fuchsia: Enable vulkan by default

Bug: 861853
Test: compile
Change-Id: Ia9eacbe863736235c64ccbdadaf3f8feaffdd3ee
Reviewed-on: https://chromium-review.googlesource.com/1234374Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592488}

gpu/vulkan/features.gni

@@ -8,7 +8,7 @@ import("//build/config/ui.gni")
 # For details see declare_args() in build/config/BUILDCONFIG.gn.
 declare_args() {
   # Enable experimental vulkan backend.
-  enable_vulkan = is_linux || is_android
+  enable_vulkan = is_linux || is_android || is_fuchsia
 
   # We want to temporarily disable Vulkan on Android to give us time to
   # investigate ways to reduce its binary size.