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The Vulkan validation layer used to error when a platform-specific
surface (in this case, a X11 surface) was used.

A build flag which enables the validation layer to recognize that
platform-specific surface was missing.

When the Vulkan validation layers moved to ANGLE they also added this
build flag. As a result, we can re-enable the validation layers on X11.

BUG=843346

Change-Id: Ifb634a60a1cd9698b5cf51e03b426a58b4387a5d
Reviewed-on: https://chromium-review.googlesource.com/c/1300335Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602961}