Clean up flag for out-of-process Viz in Mash

Multi-Process Mash by default runs Viz in Ash process. There is a
--enable-viz flag used to run it as a separate process. This CL changes
that flag to a feature named MashOopViz and adds it to chrome://flags
page. When Viz runs properly as a standalone process, we can get rid of
this flag and make it the default behavior for Mash. Then,
--in-process-gpu flag can be used to run Viz in Ash process.

BUG=862384

Change-Id: I3015a0936a7287e7a7106c86bbed55863eabdbbe
Reviewed-on: https://chromium-review.googlesource.com/c/1349478
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611985}
8 files changed