Clean up show_existing_frame decision
Clean up show_existing_frame decision

This is a followup to comments made in commit 3300552.

BUG=aomedia:2044

Change-Id: I3dba06b4919fa6b5a0b8dce658dd17825bce43f0

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