Mac gpu: on gpu<->nongpu transitions, make sure to not show uninitialized memory

Two fixes:
1. Wait for the software backing store to paint before hiding the gpu view (this was http://codereview.chromium.org/4030005 , now changed to address the review feedback)
2. Disable screen updates until window flush when hiding the gpu view

BUG=57645
TEST=Mouse over sticky notes at http://nettuts.s3.amazonaws.com/771_sticky/step5.html , shouldn't flicker. Zoom in and out on maps.google.com, shouldn't flicker.

Review URL: http://codereview.chromium.org/4148004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@64821 0039d316-1c4b-4281-b951-d872f2087c98