Enable whole program optimization for WIN64 to make build-bot green.

R=jkummerow@chromium.org
BUG=v8:2207

Review URL: https://chromiumcodereview.appspot.com/10636055

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11933 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/common.gypi

@@ -380,13 +380,16 @@
                 'FavorSizeOrSpeed': '0',
                 'OmitFramePointers': 'true',
                 'StringPooling': 'true',
-
                 'conditions': [
                   ['OS=="win" and component=="shared_library"', {
                     'RuntimeLibrary': '2',  #/MD
                   }, {
                     'RuntimeLibrary': '0',  #/MT
                   }],
+                  ['v8_target_arch=="x64"', {
+                    # TODO(2207): remove this option once the bug is fixed.
+                    'WholeProgramOptimization': 'true',
+                  }],
                 ],
               },
               'VCLinkerTool': {