Raise WebGL back buffer size limit to handle 8K monitors.

This is a short term fix to better handle larger monitors. Eliminating
the need for this limit is a more difficult task, and will follow.

Bug: 952724
Bug: 756347
Change-Id: Ia32fb6572f0888cdeb6a7dbfbe74ad382d839362
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583347
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654328}
1 file changed