Moved GL enums from GraphicsContext3D to a more generic location.

Updated all callers to use the new enums. This is the first step in eventually
removing our dependence on GraphicsContext3D's thin WebGraphicsContext3D wrapper.

BUG=326382

Review URL: https://codereview.chromium.org/93873009

git-svn-id: svn://svn.chromium.org/blink/trunk@163725 bbb929c8-8fbe-4397-9dbb-9b2b20218538