Add missing blink files to the GN build.

Source/core/core.gyp

Index: Source/core/core.gyp
diff --git a/Source/core/core.gyp b/Source/core/core.gyp
index d3bc4c83567e73144134bfa0f6e04d4717d7c9d7..bd4d3a69dc13e039287fbf2b3e5be62c845dc43c 100644
--- a/Source/core/core.gyp
+++ b/Source/core/core.gyp
@@ -732,6 +732,7 @@
       ],
     },
     {
+      # GN version: //third_party/WebKit/Source/core:testing
       'target_name': 'webcore_testing',
       'type': 'static_library',
       'dependencies': [
@@ -748,6 +749,7 @@
         'testing/v8',
       ],
       'sources': [
+        # Note: file list duplicated in GN build.
         '<@(webcore_testing_files)',
         '<(bindings_core_v8_output_dir)/V8GarbageCollectedScriptWrappable.cpp',
         '<(bindings_core_v8_output_dir)/V8GarbageCollectedScriptWrappable.h',