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Revert of Handle RenderProcessHostImpl::SetBackgrounded on Windows (patchset #4 id:60001 of https://codereview.chromium.org/926663002/)

Reason for revert:
This patch set re-enabled ChromeRenderProcessHostTest.Backgrounding, which is now failing on XP bots.

Example logs:
https://build.chromium.org/p/chromium.win/builders/XP%20Tests%20%281%29/builds/35740/steps/browser_tests%20on%20Windows-XP-SP3/logs/ChromeRenderProcessHostTest.Backgrounding
https://build.chromium.org/p/chromium.win/builders/XP%20Tests%20%281%29/builds/35740/steps/browser_tests%20on%20Windows-XP-SP3/logs/ChromeRenderProcessHostTest.Backgrounding

Original issue's description:
> Handle RenderProcessHostImpl::SetBackgrounded on Windows
>
> This was disabled on Windows because:
>
>  - http://crrev.com/274071 made it such that it was asynchronous via the renderer (as only
> the process itself can set itself in background mode on Windows).
>
>  - And then this was removed for Windows as part of issue 381820 as true background mode on
> Windows was achieved by having the renderer itself change its process priority (which
> doesn't work as the unbackgrounding request can be stuck behind other priority tasks
> running in background mode).
>
> Also, the test for this was disabled in between (see issue 394368) -- the asynchronicity
> introduced in r274071 probably causing the flakes.
>
> Overall there is no need to have the renderer enter PROCESS_MODE_BACKGROUND_BEGIN itself,
> IDLE_PRIORITY_CLASS should be sufficient as its the same as far as CPU priority is
> concerned and IO is irrelevant in the renderers.
>
> Thus we can get background mode for hidden renderers without running into issue 381820.
>
> Experiment with IDLE_PRIORITY_CLASS/BELOW_NORMAL_PRIORITY_CLASS vs the status quo.
>
> Watching the relevant perf bots (tab_switching.top_10 -> MPArch.RWH_TabSwitchPaintDuration)
> and UMA metrics MPArch.RWH_TabSwitchPaintDuration which shouldn't regress.
> And others (e.g., SessionRestore related) which will hopefully improve.
>
> Enable the experiment by default in the absence of Finch to get perf waterfall coverage.
>
> The experiment will be called "BackgroundRendererProcesses" and have 4 groups:
>  ["Disallow", "AllowBelowNormalFromBrowser",
>   "AllowIdleFromBrowser", "AllowBackgroundModeFromRenderer"]
>
> It will be a per-session (rather than per-profile) experiment, randomly assigning a user
> to a group every new Chrome session.
>
> BUG=394368, 458594
>
> Committed: https://crrev.com/aaf738a2cce69ce952386b48e3bc252bd307ef43
> Cr-Commit-Position: refs/heads/master@{#317042}

TBR=wfh@chromium.org,jam@chromium.org,gab@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=394368, 458594

Review URL: https://codereview.chromium.org/940993002

Cr-Commit-Position: refs/heads/master@{#317070}