Reorganize MediaStreamType enums

This CL does two things(although looking huge):
- Adds a new MediaStreamType to specify getDisplayMedia() requests.(PS#2)
- Renames existing MEDIA_TAB* and MEDIA_DESKTOP* enums by adding
GUM to their name to distinguish these cases that originate from
calls to extension APIs and getUserMedia().(PS#1)

Bug: 326740
Change-Id: I1c4b47f1cb6c2fed478dcd265f9b1d9525efb4cb
Reviewed-on: https://chromium-review.googlesource.com/1160000
Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Christian Fremerey <chfremer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580686}
36 files changed