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..262ba9a6d4c78d9d514db100aae9119225abd64d 100644
--- a/Source/config.gyp
+++ b/Source/config.gyp
@@ -32,6 +32,9 @@
     # 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 check GC infrastructure when compiling with clang.

[Mads Ager (chromium), 2014/01/10 09:20:50]

Maybe write this out a bit more:

Set to 1 to enable the clang plugin that checks the usage of the Blink
garbage-collection infrastructure during compilation.

?

[zerny-chromium, 2014/01/10 09:53:49]

Done.

+    # Requires building locally since GOMA doesn't yet support the plugin.
+    'blink_gc_plugin%': 0,
   },
   'targets': [
   {
@@ -98,6 +101,10 @@
             'WARNING_CFLAGS': ['-Wglobal-constructors'],
           },
         }],
+        # Only enable the blink_gc_plugin when using clang.
+        ['clang==1 and blink_gc_plugin==1', {
+          'cflags': ['<!@(../../../tools/clang/scripts/blink_gc_plugin_flags.sh)'],

[Mads Ager (chromium), 2014/01/10 09:20:50]

This is dotting its way far outside of the Blink directory. Since the chromium
and blink repos are currently separate that doesn't seem ideal. Should we move
this flag further out in the gyp structure? We have other clang plugins that are
run over the Blink sources, right? Where are they enabled? Could we move this
enabling there so it is more local?

[zerny-chromium, 2014/01/10 09:53:49]

Not as far as I can see. Currently the active plugin (FindBadConstructs) is
explicitly disabled for WebKit/blink. However, it is run on all files and the
plugin itself ignores any structures in the WebKit/blink namespace.

In our present case, it seem proper to only enable the gc plugin when compiling
blink source files (as done in this CL). Maybe we could read the plugin flags
into a blink_gc_plugin_flags gyp variable and then use that to set cflags inside
blink.

+        }],
       ],
     },
   },