Replaced --single_process with --single-process-tests

Ensured that functionality that runs tests + launcher in a
single process is consistently referred to with the switch
--single-process-tests rather than previous content::
specific --single_process in both code and documentation.

Performed basic cleanup to ensure that unused declarations
were removed and made sure switches in content:: are more
consistently declared in the appropriate headers.

Bug: 460513
Change-Id: Ic901d7af972ad43c9e0e2d31e34cc778d388d34a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848365
Reviewed-by: Mike Pinkerton <[email protected]>
Reviewed-by: Peter Kasting <[email protected]>
Commit-Queue: Thomas Lukaszewicz <[email protected]>
Cr-Commit-Position: refs/heads/master@{#705635}
18 files changed