check for null-device before using the windowless canvas
check for null-device before using the windowless canvas

Not sure why the device is null sometimes, but it correlates to playing catchup
(it seems) when resizing a window quickly.

see crbug.com/104850
Review URL: https://chromiumcodereview.appspot.com/9460006

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@123501 0039d316-1c4b-4281-b951-d872f2087c98

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