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For remote desktop environment, when Vulkan is used, VANGLE is only used
for WebGL. It is not responsible for rendering to the native windows (we
will not create GLSurfaceEGL for native windows). We will use Vulkan
swapchain to render on native windows. However some Vulkan drivers
(NVIDIA) cannot be initialized successfully, if the $DISPLAY env
variable is set, but the Xserver(remote desktop Xserver) doesn't have
necessary extensions (NV-CONTROL, NV-GLX). To workaround this issue,
this CL unsets the $DISPLAY env variable before initializing VANGLE and
sets it back after it, if the Xserver doesn't support vulkan swapchain.

Change-Id: I528a6f7d2f618123ea519f0ab4f15e788c4dd607
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424519Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809872}