Remove Frame.isMainFrame()

This patch removes redundant frame.isMainFrame() method. The suggested
way to check for main frame is:

```js
frame === page.mainFrame()
```

Fixes #120.

lib/FrameManager.js

@@ -162,7 +162,7 @@ class FrameManager extends EventEmitter {
    */
   async _evaluateOnFrame(frame, expression) {
     let contextId = undefined;
-    if (!frame.isMainFrame()) {
+    if (frame !== this._mainFrame) {
       contextId = this._frameIdToExecutionContextId.get(frame._id);
       console.assert(contextId, 'Frame does not have default context to evaluate in!');
     }
@@ -244,13 +244,6 @@ class Frame {
     return Array.from(this._childFrames);
   }
 
-  /**
-   * @return {boolean}
-   */
-  isMainFrame() {
-    return !this._detached && !this._parentFrame;
-  }
-
   /**
    * @return {boolean}
    */