Ensured that any failure to create a WebGL context dispatches a webglcontextcreationerror

BUG=352282

Review URL: https://codereview.chromium.org/214963003

git-svn-id: svn://svn.chromium.org/blink/trunk@170213 bbb929c8-8fbe-4397-9dbb-9b2b20218538