Unify request failure handling in content::UserMediaClientImpl

      Unify UserMediaClientImpl::GetUserMediaRequestFailed and
UserMediaClientImpl::GetUserMediaRequestTrackStartedFailed
and make sure all possible enum values are handled in the
switch.

BUG=459694

Review URL: https://codereview.chromium.org/937293002

Cr-Commit-Position: refs/heads/master@{#317464}