Remove busy loop in test StartupMetricsTest.ReportsValues.

Previously, the test invoked RunLoop::RunUntilIdle() in a loop. The
number of invocations of RunLoop::RunUntilIdle() was unbounded. Since
each invocation posts a delayed task (https://source.chromium.org/chromium/chromium/src/+/master:base/run_loop.cc;l=123-124;drc=fbf3c62ca96c66c95807feb8cb4478cabefb8e3c),
this could cause an OOM crash.

With this CL, the test uses RunLoop::Run() instead of
RunLoop::RunUntilIdle(). A callback is registered to exit the loop
when an histogram is recorded. The number of invocations of
RunLoop::Run() is always the same, which will prevent OOM crashes.
Also, this allows the main thread to sleep when no tasks are ready,
which reduces the test's resource consumption.

Fixed: 1152942
Change-Id: I47e5248f69e1131f034004fecae906e8803a3bdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561523
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831580}