Adds classes to implement the permissions bubble on the Mac.

PermissionBubbleCocoa is the Mac implementation of the platform-
independent PermissionBubbleView.  It creates the Cocoa implementation
when requested.

PermissionBubbleController is the Cocoa implementation for the
PermissionBubbleCocoa.  It is a UIViewController which displays the bubble,
and closes itself.

The PermissionBubbleCocoa object is owned by the BrowserWindowController,
and provided to the current PermissionBubbleManager (tied to WebContents). 
The PermissionBubbleManager then uses the implementation of
PermissionBubbleView to display the UI when required.

Also adds MockPermissionBubbleDelegate for testing, and tests for
PermissionBubbleController.

BUG=None

Review URL: https://codereview.chromium.org/151593005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254635 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254635