Simplify WebGL error message

The WebGL exception message text contains the full URL of a blocked
cross-origin resource. It should instead contain only a generic notice.

Bug: 799847
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I3a7f00462a4643c41882f2ee7e7767e6d631557e
Reviewed-on: https://chromium-review.googlesource.com/854986
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Eric Lawrence <elawrence@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528458}