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Currently, building with Vulkan support is behind a gn flag
(enable_vulkan). It would be easy to not build & test with this flag. As
a result, it would also be easy to allow Vulkan support to bitrot.

Enabling Vulkan building by default would keep the Vulkan code
buildable. This patch does that.

(You still need a command line parameter of --enable-vulkan when running
Chrome to actually use the Vulkan codepath. This is just for the
building.)

BUG=848100

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
Change-Id: I26fa03f9f6e2dc924691db913f723661b510f272
Reviewed-on: https://chromium-review.googlesource.com/1098117Reviewed-by: Brian Salomon <bsalomon@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567307}