media-internals: Differentiate error, info, and debug log messages

media-internals: Differentiate error, info, and debug log messages

This change refactors previous MediaLog::CreateMediaSourceErrorEvent()
to support general media-internals log messages that can be any one of
the new MediaLog::MediaLogLevel severities (just error, info, debug for
now).

Includes:
* Updated MEDIA_LOG and LIMITED_MEDIA_LOG macros are now additionally
  parameterized by ERROR, INFO, or DEBUG.
* Coalesced test LogCB implementations across
  source_buffer_stream_unittest, frame_processor_unittest, and
  chunk_demuxer_unittests into just one implementation in
  media/base/test_helpers.
* Differentiated the existing *MEDIA_LOG calls into the new severity
  buckets.

TEST=no regressions observed locally on linux (layout or
media_unittests), chrome://media-internals shows error, info, and debug
messages.

BUG=471902,260005

Review URL: https://codereview.chromium.org/1041353002

Cr-Commit-Position: refs/heads/master@{#323124}