Add an experimental flag FRAME_REFS_SIGNALING
Add an experimental flag FRAME_REFS_SIGNALING

This flag enables an extra bit in the frame header to indicate the
signaling choice of the reference frames in the frame header.

Change-Id: I67ef0cbb0673bf877189236b5060f2a854a05d11

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom