Add -Wnon-virtual-dtor warning for C++ code.

BUG=2659
R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/4119006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5149 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/build/common.gypi

@@ -183,6 +183,7 @@
               '-Wno-missing-field-initializers',
             ],
             'cflags_cc': [
+              '-Wnon-virtual-dtor',
               # This is enabled for clang; enable for gcc as well.
               '-Woverloaded-virtual',
             ],

webrtc/test/rtp_rtcp_observer.h

@@ -22,6 +22,7 @@ namespace test {
 
 class RtpRtcpObserver {
  public:
+  virtual ~RtpRtcpObserver() {}
   newapi::Transport* SendTransport() {
     return &send_transport_;
   }