Revert "Make //content layer default to --site-per-process mode on desktop."

This reverts commit 3b8db25cc597a642ee4b7cbaf2655c631353a799.

Reason for revert: Checking to see whether this caused https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8938549477343009904/+/steps/content_browsertests_on_Intel_GPU_on_Mac_on_Mac-10.12.6/0/logs/WebRtcAudioDebugRecordingsBrowserTest.CallWithAudioDebugRecordings/0 and similar.

Original change's description:
> Make //content layer default to --site-per-process mode on desktop.
> 
> 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: Iebd46e5502583f84a4b0a7c2ee8d8b0e081c6ff2
> Reviewed-on: https://chromium-review.googlesource.com/1153981
> Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
> Reviewed-by: Stephen Lanham <slan@chromium.org>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Reviewed-by: Bo <boliu@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#582223}

TBR=thakis@chromium.org,boliu@chromium.org,skyostil@chromium.org,alexmos@chromium.org,slan@chromium.org,lukasza@chromium.org

Change-Id: Ieca3a36422b0f90dccc0040eeece799523c954e0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 856734
Reviewed-on: https://chromium-review.googlesource.com/1171705
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582424}
20 files changed