Unbind GL context from the overlay NSView

When drawing an overlay NSView, the underlay will draw both
itself and the overlay. Leaving the GL context bound to the
overlay NSView can result in visual corruption (the desktop being
seen through where the overlay NSView is).

By unbinding the NSView, only the transparent window remains,
through which the underlay can be seen.

BUG=330701

Review URL: https://codereview.chromium.org/142083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246442 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246442