Add blink_gc_plugin variable to enable the clang plugin for checking GC consistency.

Source/config.gyp

Index: Source/config.gyp
diff --git a/Source/config.gyp b/Source/config.gyp
index 390d073ef11b61aea7f7cfed7cc2e89d2b053762..cb59e052e93d9890dd03b4f26a76b5e5769085be 100644
--- a/Source/config.gyp
+++ b/Source/config.gyp
@@ -32,6 +32,10 @@
     # If set to 1, doesn't compile debug symbols into webcore reducing the
     # size of the binary and increasing the speed of gdb.  gcc only.
     'remove_webcore_debug_symbols%': 0,
+    # Set to 1 to enable the clang plugin that checks the usage of the Blink
+    # garbage-collection infrastructure during compilation.
+    # Requires building locally since GOMA doesn't yet support the plugin.
+    'blink_gc_plugin%': 0,
   },
   'targets': [
   {
@@ -98,6 +102,10 @@
             'WARNING_CFLAGS': ['-Wglobal-constructors'],
           },
         }],
+        # Only enable the blink_gc_plugin when using clang and chrome plugins.
+        ['blink_gc_plugin==1 and clang==1 and clang_use_chrome_plugins==1', {
+          'cflags': ['<!@(../../../tools/clang/scripts/blink_gc_plugin_flags.sh)'],
+        }],
       ],
     },
   },