gn: Fix more build issues on Win

gpu/command_buffer/service/BUILD.gn

Index: gpu/command_buffer/service/BUILD.gn
diff --git a/gpu/command_buffer/service/BUILD.gn b/gpu/command_buffer/service/BUILD.gn
index 780431d388ee2732c7b981f8c5cd2e2df0af6d86..e6c58b66b3d688b00b9cc013d9fbf9a4d7b31b60 100644
--- a/gpu/command_buffer/service/BUILD.gn
+++ b/gpu/command_buffer/service/BUILD.gn
@@ -116,6 +116,9 @@ source_set("service") {
     "//third_party/khronos:khronos_headers",
   ]
 
+  # Prefer mesa GL headers to system headers, which cause problems on Win.
+  include_dirs = [ "//third_party/mesa/src/include" ]
+
   public_deps = [
     "//gpu/command_buffer/common",
   ]
@@ -134,7 +137,7 @@ source_set("service") {
   ]
 
   if (ui_compositor_image_transport) {
-    include_dirs = [ "//third_party/khronos" ]
+    include_dirs += [ "//third_party/khronos" ]
   }
 
   if (is_win || is_android || (is_linux && use_x11)) {