Force DumpRenderTree to use the Mesa backend for OpenGL as was previously done for test_shell. Verified that Mesa will be used for WebGL tests after DRT dependencies are updated.
Force DumpRenderTree to use the Mesa backend for OpenGL as was
previously done for test_shell. Verified that Mesa will be used for
WebGL tests after DRT dependencies are updated.

BUG=none
TEST=none (ran a couple of WebGL tests in DRT via new-run-webkit-tests)

Review URL: http://codereview.chromium.org/3439007

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@59866 0039d316-1c4b-4281-b951-d872f2087c98

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