Tighten about IntRect use in WebGL with overflow detection

BUG=784183
TEST=test case in the bug in ASAN build
R=kbr@chromium.org

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: Ie25ca328af99de7828e28e6a6e3d775f1bebc43f
Reviewed-on: https://chromium-review.googlesource.com/811826
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522213}