Request keyframes more frequently on stream start/decoding error.

In this CL:
 - Added FrameObject::is_keyframe() convinience function.
 - Moved logic to request keyframes on decoding error from VideoReceived to
   VideoReceiveStream.
 - Added keyframe_required as a parameter to FrameBuffer::NextFrame.

BUG=webrtc:8074

Review-Url: https://codereview.webrtc.org/2993793002
Cr-Commit-Position: refs/heads/master@{#19280}

webrtc/modules/video_coding/frame_buffer2.h

@@ -57,7 +57,8 @@ class FrameBuffer {
   //    kTimeout.
   //  - If the FrameBuffer is stopped then it will return kStopped.
   ReturnReason NextFrame(int64_t max_wait_time_ms,
-                         std::unique_ptr<FrameObject>* frame_out);
+                         std::unique_ptr<FrameObject>* frame_out,
+                         bool keyframe_required = false);
 
   // Tells the FrameBuffer which protection mode that is in use. Affects
   // the frame timing.