Enable -Winconsistent-missing-override flag.

The problem with gmock is worked around by commenting out any other override declarations in classes using gmock.

NOPRESUBMIT=True
BUG=webrtc:3970

Review-Url: https://codereview.webrtc.org/1921653002
Cr-Commit-Position: refs/heads/master@{#12563}

webrtc/modules/rtp_rtcp/source/rtcp_packet/receiver_report.h

@@ -50,7 +50,7 @@ class ReceiverReport : public RtcpPacket {
   static const size_t kRrBaseLength = 4;
   static const size_t kMaxNumberOfReportBlocks = 0x1F;
 
-  size_t BlockLength() const {
+  size_t BlockLength() const override {
     return kHeaderLength + kRrBaseLength +
            report_blocks_.size() * ReportBlock::kLength;
   }