<webview>: Context menu API implementation CL.

<webview>: Context menu API implementation CL.

Very Similar to chrome.contextMenus API, only applies to the <webview> that it is called upon.

var w = document.querySelector('webview');
w.contextMenus.OnClicked.addListener(function() { .. });
w.contextMenus.create({id: '1', title: 'one'});
w.contextMenus.update({id: '2', title: 'new', onclick: ...});
w.contextMenus.remove('1');
w.contextMenus.removeAll('1');

This CL adds the actual implementation to the previously exposed skeleton API.

Docs/snippets here:
https://docs.google.com/a/chromium.org/document/d/1AoMD6kF8ui1dikzTrwK-weVHegSqQaLV2zx4xJSh_fQ/edit

TBR=stevenjb@chromium.org
BUG=140315
Test=Can be tested inside a chrome apps <webview>.
<webview>.contextMenus.create(...) to create custom context menu items.
The API is similar to chrome.contextMenus API. Similarly .update,
.remove, .removeAll is also supported.
Click event handler can be specified as
<webview>.contextMenus.onClick.addListener(...).

Review URL: https://codereview.chromium.org/186213003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255917 0039d316-1c4b-4281-b951-d872f2087c98