Reland "*Scan: Disable *Scan if any of renderers is loading"

The browser hangs are fixed now by crrev.com/c/3116252. In addition,
this reland makes sure that the rescheduling callback is not called
under the scheduler lock in
MUAwareTaskBasedBackend::NeedsToImmediatelyScan() (as otherwise this can
result in a deadlock if the callback frees anything).

For sheriffs: please note that the CL might cause hangs. We are not
aware of any though.

Original change's description:
> *Scan: Disable *Scan if any of renderers is loading
>
> In scenarios when a renderer loads a page while *Scan is actively
> scanning the browser heap, it can happen that *Scan takes away all the
> machine cores. This slows down page loading and causes some metrics to
> regress ("First Input Delay", "Time to First Contentful Paint", etc).
>
> This CL introduces StarScanLoadObserver that listens to load changes in
> each WebContents using WebContentsObserver API. In case any of the
> WebContents has renderers that are loading a new document, *Scan in the
> browser process temporarily disables *Scan until all the renderers
> finish loading.
>
> This (intrusive) change is a temporary experiment. Ideally the jobs API
> would be used for scanning tasks, however, due to the cyclic dependency
> between the allocator and the scheduler, it's not trivial to switch to
> jobs API. The bug 1231679 aims to reevaluate the entire approach.
>
> Bug: 11297512,1231679
> Change-Id: Idc75a9d4a8f8bc608cc0934f43948582c0457cd8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3015346


> Commit-Queue: Anton Bikineev <bikineev@chromium.org>
> Reviewed-by: Charlie Reis <creis@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#906310}

> Bug: 11297512,1231679

Change-Id: I46086caa28a57dd330511d38c129efed660ccc14
Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3114291


Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915104}