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Previously, GLSL validation was implicitly enabled on contexts that had
the noExtensions flag on. Use the webGL flag instead, making the code
less fragile.

BUG=397183
TEST=WebGL conformance tests

Review URL: https://codereview.chromium.org/421513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285887 0039d316-1c4b-4281-b951-d872f2087c98