Add a GN argument to control whether to use_dawn

use_dawn controls whether Dawn and its dependencies are integrated in
Chromium to support WebGPU. It currently defaults to false while we work
out the details of the integration of Dawn both code-wise and for the
launch.

BUG=877147

Change-Id: I31c3780e024f325c6dbcd4b89dd809199de8df0d
Reviewed-on: https://chromium-review.googlesource.com/c/1488921Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635577}

gpu/command_buffer/service/BUILD.gn

@@ -6,6 +6,7 @@ import("//build/config/jumbo.gni")
 import("//build/config/ui.gni")
 import("//gpu/vulkan/features.gni")
 import("//third_party/protobuf/proto_library.gni")
+import("//ui/gl/features.gni")
 
 group("service") {
   if (is_component_build) {
@@ -324,6 +325,10 @@ target(link_target_type, "gles2_sources") {
     }
   }
 
+  if (use_dawn) {
+    deps += [ "//third_party/dawn:libdawn_native" ]
+  }
+
   if (is_mac) {
     # Required by gles2_cmd_decoder.cc on Mac.
     libs = [

ui/gl/BUILD.gn

@@ -35,6 +35,7 @@ buildflag_header("buildflags") {
   use_egl_on_mac = use_egl && is_mac
   flags = [
     "ENABLE_SWIFTSHADER=$enable_swiftshader",
+    "USE_DAWN=$use_dawn",
     "USE_EGL_ON_MAC=$use_egl_on_mac",
     "USE_STATIC_ANGLE=$use_static_angle",
   ]

ui/gl/features.gni

@@ -10,4 +10,9 @@ declare_args() {
   # Should EGL support be compiled. Can be overriden to test during bring up
   # of EGL support on other platforms
   use_egl = is_win || is_android || is_linux || is_fuchsia || is_mac
+
+  # Should Dawn support be compiled to back the WebGPU implementation.
+  # Also controls linking Dawn depdencies in such as shaderc, SPIRV-Tools and
+  # SPIRV-Cross
+  use_dawn = false
 }