Introduce setup_buffer_pool() method.
Introduce setup_buffer_pool() method.

This refactors the duplicate code in setup_frame_size() and
setup_frame_size_with_refs().

BUG=aomedia:2044

Change-Id: Idab9b9ff03c72f81926ea54bef0cbb31db754d07

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