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[RenderProcessHost/Desktop] Update process priority on process launch and fix navigation priority inversion

This is a second take on
https://chromium-review.googlesource.com/c/chromium/src/+/754299

It's under an experiment to (1) be able to see the side-effects and (2)
make merge to M69 easier. Intentionally kept old (and buggy) behavior
intact under the non-experimental branch for comparison.

This fixes all aspects of crbug.com/560446:
 1) Fixes the OP better than r385608 did.
 2) Fixes the side-effect of r385608 which was to default all tabs
    created backgrounded to run at foreground priority until visiblity
    was toggle on/off again.
 3) Fixes a major priority inversion on foreground navigation which
    could cause them to background their associated renderer while
    navigation (which reintroduced the OP but worse since all background
    tabs were running foregrounded per r385608)...
    See https://crbug.com/560446#c74 for details.

Updated ChildProcessLauncherPriority::is_background() to consistently
reflect backgrounding decision to all callers and renamed
|ChildProcessLauncherPriority::foreground| to
|ChildProcessLauncherPriority::visible| to better reflect that it's a
property but not the resulting foreground/background state.

See in code comment on ShouldBoostPriorityForPendingViews() for more
details.

TBR=haraken@chromium.org (side-effect in third_party/blink)

Bug: 560446
Change-Id: I901b702506a44704b53c007bcdd498fb60824e94
Reviewed-on: https://chromium-review.googlesource.com/1142593
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585918}