Factor out repeated CPU measurement code

This moves CPUMeasurementDelegate to its own header file, so that it can
still be exposed to tests once CPUMeasurementMonitor becomes private,
and moves SimulatedCPUMeasurementDelegate out of individual tests into a
shared file in test_support. Code to keep track of simulated delegates
is moved from the test fixtures into a new
SimulatedCPUMeasurementDelegateFactory.

Also simplifies some tests to use the RunInGraph() helper instead of
defining their own RunLoop helpers.

R=agale

Bug: 1471683
Change-Id: Iec39bc77182784e0e3eecb7dfe731c61bc9431a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4950270
Commit-Queue: Joe Mason <joenotcharles@google.com>
Reviewed-by: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1211676}