Move FALLTHROUGH macro to a separate header, and give it an RTC_ prefix

Bug: chromium:805946
Change-Id: Ibb5dce9af27d0e48c9aee6b0a860b6f62b3c76a0
Reviewed-on: https://webrtc-review.googlesource.com/46145
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21889}

modules/video_coding/jitter_buffer.cc

@@ -25,6 +25,7 @@
 #include "modules/video_coding/packet.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/system/fallthrough.h"
 #include "rtc_base/trace_event.h"
 #include "system_wrappers/include/clock.h"
 #include "system_wrappers/include/event_wrapper.h"
@@ -749,7 +750,7 @@ VCMFrameBufferEnum VCMJitterBuffer::InsertPacket(const VCMPacket& packet,
           frame_event_->Set();
         }
       }
-      FALLTHROUGH();
+      RTC_FALLTHROUGH();
     }
     // Note: There is no break here - continuing to kDecodableSession.
     case kDecodableSession: {