feat: expose frame's execution contexts (#3048)

This patch exposes frame's execution contexts, making it possible
to debug extension's content scripts.

This is a resurrected #2812.

lib/ExecutionContext.js

@@ -30,9 +30,25 @@ class ExecutionContext {
     this._client = client;
     this._frame = frame;
     this._contextId = contextPayload.id;
+    this._isDefault = contextPayload.auxData ? !!contextPayload.auxData['isDefault'] : false;
+    this._name = contextPayload.name;
     this._objectHandleFactory = objectHandleFactory;
   }
 
+  /**
+   * @return {string}
+   */
+  name() {
+    return this._name;
+  }
+
+  /**
+   * @return {boolean}
+   */
+  isDefault() {
+    return this._isDefault;
+  }
+
   /**
    * @return {?Puppeteer.Frame}
    */