Introduces a new media::cast::FrameId class to represent unique frame
IDs throughout the implementation.  This replaces a number of mixed uses
of both uint32_t and uint8_t throughout the implementation, and resolves
a few bugs that were exposed as a result.

Bugs fixed:

1. In media/cast/.../framer.cc and media/cast/cast_message_builder.cc:
Removal of dead code, fixed redundant look-up-then-erase iterations on
underlying map data structure, fixed memory leaks caused by not erasing
old map entries.

2. In media/cast/rtcp_utility.cc: Fixed 8-bit frame ID expansion to be
more aware of old packets "posing" as "packets from the future."  A
protocol spec addition is in the works to fully resolve the "8 bits is
not enough" issue in a future change.

BUG=530845,530839
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1576943006

Cr-Commit-Position: refs/heads/master@{#389334}