feat: Attribute network requests to frames (#1646)

This patch introduces `request.frame()` method that returns the frame that
initiated request.

Fixes #1579

docs/api.md

@@ -179,6 +179,7 @@
   * [request.abort([errorCode])](#requestaborterrorcode)
   * [request.continue([overrides])](#requestcontinueoverrides)
   * [request.failure()](#requestfailure)
+  * [request.frame()](#requestframe)
   * [request.headers()](#requestheaders)
   * [request.method()](#requestmethod)
   * [request.postData()](#requestpostdata)
@@ -2061,6 +2062,9 @@ page.on('requestfailed', request => {
 });
 ```
 
+#### request.frame()
+- returns: <?[Frame]> A matching [Frame] object, or `null` if navigating to error pages.
+
 #### request.headers()
 - returns: <[Object]> An object with HTTP headers associated with the request. All header names are lower-case.