Roll ANGLE from 6d781655d0f3 to 6ee18c2d01f6 (3 revisions)

https://chromium.googlesource.com/angle/angle.git/+log/6d781655d0f3..6ee18c2d01f6

2020-06-25 syoussefi@chromium.org Vulkan: Remove debug symbols from internal shaders
2020-06-25 jonahr@google.com CGL: Add basic multithreading support
2020-06-25 jonahr@google.com Add basic multithreading support to GLX

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/angle-chromium-autoroll
Please CC cwallez@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

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-asan;luci.chromium.try:win_optional_gpu_tests_rel;luci.chromium.try:linux-swangle-try-x64;luci.chromium.try:win-swangle-try-x86
Bug: chromium:1087084
Tbr: cwallez@google.com
Change-Id: I2d889ea211a85ce2016fe14a0c0dc0c7e57e7a23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2268501Reviewed-by: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Commit-Queue: chromium-autoroll <chromium-autoroll@skia-public.iam.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#782807}

DEPS

@@ -200,7 +200,7 @@ 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': '6d781655d0f3cd76e3b207c15baca8c2f51b15d2',
+  'angle_revision': '6ee18c2d01f6c0cca06fc21c5c5ed63bf168e6c9',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.