Roll src/third_party/angle/ a4fa9c278..aa8b3982a (4 commits)

https://chromium.googlesource.com/angle/angle.git/+log/a4fa9c2782da..aa8b3982aeeb

$ git log a4fa9c278..aa8b3982a --date=short --no-merges --format='%ad %ae %s'
2018-04-16 tobine Roll (1/2) LVL version forward and disable VANGLE
2018-04-16 jmadill Vulkan: Fix texture descriptor set alloc count.
2018-04-15 jmadill Optimize ValidateDrawAttribs: Part 1.
2018-04-13 jmadill Vulkan: Rename StagingStorage to PixelBuffer.

Created with:
  roll-dep src/third_party/angle


The AutoRoll server is located here: https://angle-chromium-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.


CQ_INCLUDE_TRYBOTS=luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
TBR=jmadill@chromium.org
No-try: True

Change-Id: Ie22e822e0e4688b255c641ca2c3ea378f4f843dc
Reviewed-on: https://chromium-review.googlesource.com/1014465
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: angle-chromium-autoroll <angle-chromium-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#551162}

DEPS

@@ -98,7 +98,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling ANGLE
   # and whatever else without interference from each other.
-  'angle_revision': 'a4fa9c2782dabfa279c6fa2bb3cd29dfa4b1ebdd',
+  'angle_revision': 'aa8b3982aeeb90c9d4f818117a6be1a6904def5a',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
   # and whatever else without interference from each other.