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.

docs/api.md

@@ -87,7 +87,6 @@
     + [frame.evaluate(pageFunction, ...args)](#frameevaluatepagefunction-args)
     + [frame.hover(selector)](#framehoverselector)
     + [frame.isDetached()](#frameisdetached)
-    + [frame.isMainFrame()](#frameismainframe)
     + [frame.name()](#framename)
     + [frame.parentFrame()](#frameparentframe)
     + [frame.url()](#frameurl)
@@ -837,11 +836,6 @@ browser.newPage().then(async page =>
 
 Returns `true` if the frame has being detached, or `false` otherwise.
 
-#### frame.isMainFrame()
-- returns: <[boolean]>
-
-Returns `true` is the frame is page's main frame, or `false` otherwise.
-
 #### frame.name()
 - returns: <[string]>
 

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}
    */