<webview>: Implement default action for permissions

This CL does not change developer-facing functionality, but merely tweaks the internal webview API.

1. WebView.setPermission now takes in an enum of ['allow', 'deny', 'default'] instead of a boolean for allowed.
2. The userInput field of WebView.setPermission is now optional for brevity.
3. A callback is added as a last parameter to WebView.setPermission that indicates whether the action was actually performed. Warnings are only shown if an action is denied because the developer did not take an action.
4. There are no longer any references to the experimental permission loadplugin in web_view.js

BUG=306221
Test=WebViewTest.*, WebViewInteractiveTest.* all pass as expected.

Review URL: https://codereview.chromium.org/25885002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228879 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228879