Enable -Woverloaded-virtual for gcc.

BUG=
TEST=build on Linux/gcc.

Review URL: https://webrtc-codereview.appspot.com/417001

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1790 4adac7df-926f-26a2-2b94-8c16560cd09d

src/build/common.gypi

@@ -118,6 +118,10 @@
               '-Wno-unused-parameter',
               '-Wno-missing-field-initializers',
             ],
+            'cflags_cc': [
+              # This is enabled for clang; enable for gcc as well.
+              '-Woverloaded-virtual',
+            ],
           }],
         ],
       }],