Do not call OnDecoderTiming before timing values are set.

Wait until first frame is decoded to avoid include zeros in stats.

BUG=b/32659204

Review-Url: https://codereview.webrtc.org/2582313002
Cr-Commit-Position: refs/heads/master@{#15752}
diff --git a/webrtc/modules/video_coding/timing.h b/webrtc/modules/video_coding/timing.h
index 85a3dd0..e7d2b1f 100644
--- a/webrtc/modules/video_coding/timing.h
+++ b/webrtc/modules/video_coding/timing.h
@@ -92,8 +92,9 @@
   // certain amount of processing time.
   bool EnoughTimeToDecode(uint32_t available_processing_time_ms) const;
 
-  // Return current timing information.
-  void GetTimings(int* decode_ms,
+  // Return current timing information. Returns true if the first frame has been
+  // decoded, false otherwise.
+  bool GetTimings(int* decode_ms,
                   int* max_decode_ms,
                   int* current_delay_ms,
                   int* target_delay_ms,