Remove aura enum from DesktopMediaID to fix desktop mirroring audio (CrOS).
This change removes the TYPE_AURA_WINDOW enum value from
content::DesktopMediaID, and requires the use of TYPE_WINDOW or
TYPE_SCREEN where appropriate. This allows the logic that determines
whether to capture and mirror system-wide audio to work on CrOS.
Also cleaned up code in desktop_media_id.cc to reduce memory usage by
replacing base::Singleton with base::LazyInitializer and two std::maps
with a single std::vector (since there should always be fewer than
hundreds of entries).
BUG=468639,513093
TEST=Use Google Cast Extension to mirror the desktop to a Chromecast, play audio, and expect the audio to be heard from the Chromecast.
Review URL: https://codereview.chromium.org/
1255573002
Cr-Commit-Position: refs/heads/master@{#341653}