The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/78cf93348c353ab9dd7454c3e85ce4f199a5568e

commit 78cf93348c353ab9dd7454c3e85ce4f199a5568e
Author: John Rummell <jrummell@chromium.org>
Date: Thu Aug 17 21:52:51 2017

Add UMA result for failed EME session creation

MediaKeySession.generateRequest() and MediaKeySession.load() internally use
promises that return the result as a string (session_id). Validation of the
returned |session_id| may result in generateRequest() failing, or in false
being returned by load(), indicating that the session to be loaded can't be
found. Current UMA logging of the result always reports SUCCESS. Add
additional values to the UMA logged to indicate SESSION_NOT_FOUND or
SESSION_ALREADY_EXISTS.

BUG= 756109 
TEST=tested locally

Change-Id: Ic8134efcbc95f2fd468dfe73439a6f1d5d6ababa
Reviewed-on: https://chromium-review.googlesource.com/619408
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495334}
[modify] https://crrev.com/78cf93348c353ab9dd7454c3e85ce4f199a5568e/media/blink/cdm_result_promise_helper.h
[modify] https://crrev.com/78cf93348c353ab9dd7454c3e85ce4f199a5568e/media/blink/new_session_cdm_result_promise.cc
[modify] https://crrev.com/78cf93348c353ab9dd7454c3e85ce4f199a5568e/tools/metrics/histograms/enums.xml

Marking this as fixed, although it may take a few days before we see any data.

generateRequest() promise succeeds if SUCCESS reported. SESSION_NOT_FOUND and SESSION_ALREADY_EXISTS end up as an InvalidStateError rejection, so those three buckets can be combined.

load() promise succeeds with true if SUCCESS reported. If SESSION_NOT_FOUND reported, load() promise succeeds with false returned (meaning the session can't be loaded since it doesn't exist). SESSION_ALREADY_EXISTS ends up as an InvalidStateError rejection.