Fixes crash when switching to new tabs containing audio/video elements.
Fixes crash when switching to new tabs containing audio/video elements.

Cause was we're calling platformContext(), one of the few GraphicsContext operations that asserts if painting is disabled.

BUG=19677,20138
TEST=open a video in a new background tab, wait a bit, then switch to it -- shouldn't crash

Review URL: http://codereview.chromium.org/183038


git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@25125 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,ropik/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,adobe/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,ropik/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,gavinp/chromium,Crystalnix/house-of-life-chromium,yitian134/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,Crystalnix/house-of-life-chromium,Crystalnix/house-of-life-chromium,yitian134/chromium