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_object.h

@@ -44,6 +44,8 @@ class FrameObject : public webrtc::VCMEncodedFrame {
 
   size_t size() const { return _length; }
 
+  bool is_keyframe() const { return num_references == 0; }
+
   // The tuple (|picture_id|, |spatial_layer|) uniquely identifies a frame
   // object. For codec types that don't necessarily have picture ids they
   // have to be constructed from the header data relevant to that codec.