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..e39ce97affb2f1184f71c30e5971e0714f6fd481 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 and OS!="win"', {

[Mads Ager (chromium), 2014/01/10 10:50:45]

Is the OS!="win" needed? If we start using clang on Windows it seems that we
should use the plugins as well.

Do we want the use_chrome_plugins part? Do we rely on anything that is only
there when clang_use_chrome_plugins is set?

[zerny-chromium, 2014/01/10 11:33:59]

Currently build/common.gypi [1] only defines the plugin flags when not on
windows. I'm not sure that the setup actually works if clang==1 and OS=="win"
since there are unguarded uses of the variables being defined under the
assumption that OS!="win".

I'll remove the OS!="win" condition here, since if clang_blink_gc_plugin_flags
is properly defined for windows then this code works too.

The clang_use_chrome_plugins variable is default enabled but is used to disable
plugins if need be, eg, when running the type profiler which is a different
clang build IFAICT.

I think we should keep that condition because if the exisiting plugin does not
work with the type profiler then I doubt this one will.

[1] https://codereview.chromium.org/133463002/diff/70001/build/common.gypi

+          'cflags': ['<@(clang_blink_gc_plugin_flags)'],
+        }],
       ],
     },
   },