This improves the level of information available for web authors to use
when debugging media playback errors.

* Adds support for the MediaError.message field [1] [2]

* Changes MediaLog::PipelineStatusToString() to return just a
  stringified version of the enum identifier, suitable for use as a
  UA-specific-code when included in MediaError.message.

* Changes MediaLog::GetErrorMessage() to produce messages in the form:
  [[Stringified pipeline error code, if any, without any colon][: The
  first MEDIA_ERROR_LOG_ENTRY from the log, if any, since that is most
  likely to contain the most precise error information]]
  The produced message does not contain any newlines.
  See the updated media_browsertest.cc for basic examples.

* When the message would otherwise be empty for various cases in
  HTMLMediaElement, and more meaningful information beyond the basic
  standard MediaError codes and HTMLMediaElement networkStates is
  available, reports basic additional information (prefixed by error
  code "MEDIA_ELEMENT_ERROR: ".) This should assist edge case
  differentiation. See the updated media_browsertest.cc for basic
  examples.

* Test coverage:
  * Adds basic layout test coverage test similar to that added in [3].
  * Adds basic content browser test coverage for a small variety of
    errors, since these messages' format and content are specific to
    Chrom*.

The format of the message string may change in future (for example, see
discussion in [4]). Also, the variety of error codes in the message
prior to the first ':' will likely change in future, and the descriptive
error message detail following the ':' will likely change to include
more cases or more consistent format in future.

[1] - https://github.com/whatwg/html/issues/2085
[2] - https://github.com/whatwg/html/pull/2086
[3] - https://github.com/w3c/web-platform-tests/pull/4638
[4] - https://github.com/whatwg/html/issues/2531
[Intent to implement and ship] -
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/yDIyWozG1xk

BUG=601086, 710617

Review-Url: https://codereview.chromium.org/2660003003
Cr-Commit-Position: refs/heads/master@{#466359}