Fixed issue where multiple WebGL contexts could not be created in some circumstances.

BUG=319265
TEST=run the following command line: "content/test/gpu/run_gpu_test webgl_conformance --webgl-conformance-version=1.0.3 --page-filter=context-creation.html"

Review URL: https://codereview.chromium.org/99963005

git-svn-id: svn://svn.chromium.org/blink/trunk@163148 bbb929c8-8fbe-4397-9dbb-9b2b20218538