The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a28f6b885ca573d110057c85ba718a45e1d84037

commit a28f6b885ca573d110057c85ba718a45e1d84037
Author: Brandon Jones <bajones@chromium.org>
Date: Mon Jul 16 16:26:49 2018

Fix WebXR framebuffer discard/clear logic.

In cases when we explicitly resolved multisampled framebuffers the
resolved framebuffer was being discarded instead of the drawing
framebuffer. This likely had no ill effects but was not giving us the
benefit expected on those devices.

Additionally, framebuffers were not clearing prior to each new frame
(although discard would sometimes appear to have that effect), which
would result in data from previous frames being left behind in the
framebuffer in an unpredictable manner. This patch introduces a
consistent clear for predictability.

Bug:  863242 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib78c7d02812574a655220c6bdba50e02904dd483
Reviewed-on: https://chromium-review.googlesource.com/1135943
Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575297}
[modify] https://crrev.com/a28f6b885ca573d110057c85ba718a45e1d84037/third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.cc
[modify] https://crrev.com/a28f6b885ca573d110057c85ba718a45e1d84037/third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.h