Implement public API coverage

This patch:
- implements a basic public API coverage based on 'helper.tracePublicAPI' methods
- adds `npm run coverage` command which reports coverage after running all of the unit tests

References #50.

lib/helper.js

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+/** @type {?Map<string, boolean>} */
+let apiCoverage = null;
 class Helper {
   /**
    * @param {function()|string} fun
@@ -116,16 +118,21 @@ class Helper {
     let className = classType.prototype.constructor.name;
     className = className.substring(0, 1).toLowerCase() + className.substring(1);
     const debug = require('debug')(`puppeteer:${className}`);
-    if (!debug.enabled)
+    if (!debug.enabled && !apiCoverage)
       return;
     for (let methodName of Reflect.ownKeys(classType.prototype)) {
       const method = Reflect.get(classType.prototype, methodName);
       if (methodName === 'constructor' || typeof methodName !== 'string' || methodName.startsWith('_') || typeof method !== 'function')
         continue;
+      if (apiCoverage)
+        apiCoverage.set(`${className}.${methodName}`, false);
       Reflect.set(classType.prototype, methodName, function(...args) {
         let argsText = args.map(stringifyArgument).join(', ');
         let callsite = `${className}.${methodName}(${argsText})`;
-        debug(callsite);
+        if (debug.enabled)
+          debug(callsite);
+        if (apiCoverage)
+          apiCoverage.set(`${className}.${methodName}`, true);
         return method.call(this, ...args);
       });
     }
@@ -163,6 +170,17 @@ class Helper {
       listener.emitter.removeListener(listener.eventName, listener.handler);
     listeners.splice(0, listeners.length);
   }
+
+  /**
+   * @return {?Map<string, boolean>}
+   */
+  static publicAPICoverage() {
+    return apiCoverage;
+  }
+
+  static recordPublicAPICoverage() {
+    apiCoverage = new Map();
+  }
 }
 
 module.exports = Helper;