Fix some GN component build issues.

gpu/BUILD.gn

Index: gpu/BUILD.gn
diff --git a/gpu/BUILD.gn b/gpu/BUILD.gn
index 75aed69171511ca9eb370ff84b1e7834ea4f2d57..c7f70eb7e5adc30e11db0457ee988a2e5ff0dd1d 100644
--- a/gpu/BUILD.gn
+++ b/gpu/BUILD.gn
@@ -36,6 +36,10 @@
 import("//testing/test.gni")
 import("//build/config/ui.gni")
 
+config("gpu_implementation") {
+  defines = [ "GPU_IMPLEMENTATION" ]
+}
+
 component("gpu") {
   public_deps = [
     "//gpu/command_buffer/client",
@@ -56,6 +60,8 @@ source_set("test_support") {
     "command_buffer/service/gles2_cmd_decoder_mock.cc",
   ]
 
+  configs += [ ":gpu_implementation" ]
+
   public_deps = [
     ":gpu",
     "//gpu/command_buffer/client:gles2_interface",