Fix a crash in GLHelper when the context is lost.
Fix a crash in GLHelper when the context is lost.

BUG=chromium-os:28909
TEST=Locally modify the thumbnailer to generate thumbnail on every tab swithcing, open a tab, swich tabs (to ensure GLHelper is instantiated), kill GPU process twice, and checked chrome didn't crash

Review URL: https://chromiumcodereview.appspot.com/9965123

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@130524 0039d316-1c4b-4281-b951-d872f2087c98

adobe/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,ropik/chromium,adobe/chromium,ropik/chromium,adobe/chromium,adobe/chromium