[reland] Make //content layer default to --site-per-process mode on desktop.

This is a reland of unmodified r582223, after separately taking care of
https://crbug.com/873780 which caused the earlier revert.  Original CL
description follows below:

After this CL site-per-process is the default in all layers on desktop
platforms, except:

- Layout Tests which still run with no isolation by default
  (see https://crbug.com/856734#c5 for the explanation why)

- //content embedders that don't want to use site-per-process: //chromecast

- //content embedders that don't yet support site-per-process, but will
  need to migrate eventually: //headless

Also note that even after this CL:

- //content on Android still defaults to no isolation.
  This is compatible with not_site_per_process_* test steps
  because such steps are not run on Android bots...

- //chrome layer ChromeContentBrowserClient controls usage of
  site-per-process in the Chrome browser and continues to be controlled
  by a field trial (note that Android is not covered by the field trial
  or by testing/variations/fieldtrial_testing_config.json).

Bug: 856734
Change-Id: I0cfd7e342c44a36d22a885b46892a9e4e3f2c758
Tbr: Stephen Lanham <slan@chromium.org>
Tbr: Sami Kyöstilä <skyostil@chromium.org>
Tbr: Bo <boliu@chromium.org>
Tbr: Nico Weber <thakis@chromium.org>
Tbr: Alex Moshchuk <alexmos@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1174855
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583047}
20 files changed