HistoryTimer's can't run in the background because they use a timer
with a simple api of Start() and Stop(). This CL fixes this problem
by building a base class TimedHistogram that doesn't have a timer.

The class HistoryTimer is modified to use this base class so that
uses that run on the foreground thread do not need to be modified.

It also adds a new class TimedHistogramScope that defines the timer
in this class. This allows the corresopnding TimedHistogram class to
be type safe.

BUG=v8:6361

Review-Url: https://codereview.chromium.org/2929853003
Cr-Commit-Position: refs/heads/master@{#46150}