Roll WebGL 34842fa..12192b9

https://chromium.googlesource.com/external/khronosgroup/webgl.git/+log/34842fa..12192b9

BUG=765469, 768969, 769989, 772651

TEST=bots

TBR=zmo@chromium.org, kainino@chromium.org

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

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: I180314943cb6427b916790f5ae5bf295c87330ea
Reviewed-on: https://chromium-review.googlesource.com/764818
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516218}