Add skia_enable_discrete_gpu argument to GN

This forces all of our testing tools to run with the discrete GPU in
laptop systems that have that option.

BUG=skia:

Change-Id: Ibd7629d6de5f063cdf219b3c7469210af5085d90
Reviewed-on: https://skia-review.googlesource.com/6474
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 102b785..24651c7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -26,6 +26,7 @@
   skia_use_zlib = true
 
   skia_enable_android_framework_defines = false
+  skia_enable_discrete_gpu = true
   skia_enable_gpu = true
   skia_enable_tools = is_skia_standalone
   skia_enable_vulkan_debug_layers = is_skia_standalone && is_debug
@@ -148,6 +149,9 @@
   if (skia_use_angle) {
     defines += [ "SK_ANGLE" ]
   }
+  if (skia_enable_discrete_gpu) {
+    defines += [ "SK_ENABLE_DISCRETE_GPU" ]
+  }
 }
 
 # Any code that's linked into Skia-the-library should use this config via += skia_library_configs.