feat: implement CSS Coverage (#1714)

This patch adds two new methods to the `page.coverage` namespace:
- `page.coverage.startCSSCoverage()` - to initiate css coverage
- `page.coverage.stopCSSCoverage()` - to stop css coverage

The coverage format is consistent with the JavaScript coverage.

docs/api.md

@@ -199,7 +199,9 @@
   * [target.type()](#targettype)
   * [target.url()](#targeturl)
 - [class: Coverage](#class-coverage)
+  * [coverage.startCSSCoverage(options)](#coveragestartcsscoverageoptions)
   * [coverage.startJSCoverage(options)](#coveragestartjscoverageoptions)
+  * [coverage.stopCSSCoverage()](#coveragestopcsscoverage)
   * [coverage.stopJSCoverage()](#coveragestopjscoverage)
 
 <!-- tocstop -->
@@ -2155,11 +2157,56 @@ Identifies what kind of target this is. Can be `"page"`, `"service_worker"`, or
 
 ### class: Coverage
 
+Coverage gathers information about parts of JavaScript and CSS that were used by the page.
+
+An example of using JavaScript and CSS coverage to get percentage of initially
+executed code:
+
+```js
+// Enable both JavaScript and CSS coverage
+await Promise.all([
+  page.startJSCoverage(),
+  page.startCSSCoverage()
+]);
+// Navigate to page
+await page.goto('https://example.com');
+// Disable both JavaScript and CSS coverage
+const [jsCoverage, cssCoverage] = await Promise.all([
+  page.stopJSCoverage(),
+  page.stopCSSCoverage(),
+]);
+let totalBytes = 0;
+let usedBytes = 0;
+const coverage = [].concat(jsCoverage, cssCoverage);
+for (const entry of coverage) {
+  totalBytes += entry.text.length;
+  for (const range of entry.ranges)
+    usedBytes += range.end - range.start - 1;
+}
+console.log(`Bytes used: ${usedBytes / totalBytes * 100}%`);
+```
+
+#### coverage.startCSSCoverage(options)
+- `options` <[Object]>  Set of configurable options for coverage
+  - `resetOnNavigation` <[boolean]> Whether to reset coverage on every navigation. Defaults to `true`.
+- returns: <[Promise]> Promise that resolves when coverage is started
+
 #### coverage.startJSCoverage(options)
 - `options` <[Object]>  Set of configurable options for coverage
   - `resetOnNavigation` <[boolean]> Whether to reset coverage on every navigation. Defaults to `true`.
 - returns: <[Promise]> Promise that resolves when coverage is started
 
+#### coverage.stopCSSCoverage()
+- returns: <[Promise]<[Array]<[Object]>>> Promise that resolves to the array of coverage reports for all stylesheets
+  - `url` <[string]> StyleSheet URL
+  - `text` <[string]> StyleSheet content
+  - `ranges` <[Array]<[Object]>> StyleSheet ranges that were used. Ranges are sorted and non-overlapping.
+    - `start` <[number]> A start offset in text, inclusive
+    - `end` <[number]> An end offset in text, exclusive
+
+> **NOTE** CSS Coverage doesn't include dynamically injected style tags without sourceURLs.
+
+
 #### coverage.stopJSCoverage()
 - returns: <[Promise]<[Array]<[Object]>>> Promise that resolves to the array of coverage reports for all non-anonymous scripts
   - `url` <[string]> Script URL
@@ -2171,22 +2218,6 @@ Identifies what kind of target this is. Can be `"page"`, `"service_worker"`, or
 > **NOTE** JavaScript Coverage doesn't include anonymous scripts; however, scripts with sourceURLs are
 reported.
 
-An example of using JavaScript coverage to get percentage of executed
-code:
-
-```js
-await page.startJSCoverage();
-await page.goto('https://example.com');
-const coverage = await page.stopJSCoverage();
-let totalBytes = 0;
-let usedBytes = 0;
-for (const entry of coverage) {
-  totalBytes += entry.text.length;
-  for (const range of entry.ranges)
-    usedBytes += range.end - range.start - 1;
-}
-console.log(`Coverage is ${usedBytes / totalBytes * 100}%`);
-```
 
 [Array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array "Array"
 [boolean]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type "Boolean"