compute pressure: Calculate ownContributionEstimate

`ownContributionEstimate` provides the information on how much the
rendering process is contributing to the current compute pressure.
It is only an estimate.

This path is the first patch of a two patches patchset.

This patch implements the implementation specific part of spec
changes[1]. It includes the changes in //services and //content

OwnContributionEstimate is calculated in //content/browser, because
it is a logical place where we can have the concept of process.
To calculate the own contribution estimate, the global cpu_utilization,
from service::mojom::PressureUpdate is used as well as the cumulative
CPU usage for the process under scrutiny.
The cumulative CPU usage is available by using Process::ProcessMetrics
class.

The ownContributionEstimate value exposition to //Blink or the WebAPI,
as well as the CDP and testdriver changes are addressed in the second
patch.

[1] https://w3c.github.io/compute-pressure/?experimental=1#the-owncontributionestimate-attribute

Bug: 402033762
Change-Id: I63af1835e011297669859f68b78300f69e46857d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6309002
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Arnaud Mandy <arnaud.mandy@intel.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458748}