Drop "trials" from ...DisableSiteIsolationTrials... where backcompatible

This CL edits descriptions and comments related to the
--disable-site-isolation-trials switch.  The switch is implemented and
used in a way that disables not only site isolation trials, but also
opts out of the default site isolation mode (i.e. out of
site-per-process mode that is the default on desktop since M67).

The CL avoids editing the *name* of the switch to avoid backcompatibility
problems around chrome://flags and/or test tools.  There are no intended
behavior changes except for different description texts for the
enterprise policy and chrome://flags.

Change-Id: Ia2076c4120d159f9d80e7d578ea0e673fdfda759
Bug: 901522
Reviewed-on: https://chromium-review.googlesource.com/c/1315535
Commit-Queue: Ɓukasz Anforowicz <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Julian Pastarmov <[email protected]>
Reviewed-by: Nasko Oskov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#605409}
12 files changed