Added failIfMajorPerformanceCaveat WebGL context creation attribute

See http://www.khronos.org/registry/webgl/specs/latest/1.0/#5.2.1 for further details.

Won't actually be active until the attribute flag is handled in Chrome

BUG=319965

Review URL: https://codereview.chromium.org/76663004

git-svn-id: svn://svn.chromium.org/blink/trunk@162398 bbb929c8-8fbe-4397-9dbb-9b2b20218538