Currently, we only report UMA statistics on receiving a pressure
change event from the system. This means that we could be missing
many ticks of "normal" pressure from sessions that never encounter
pressure, skewing our stats. Other platforms don't have this issue
since they poll.

This change piggybacks statistic reporting on GetCurrentPressureLevel()
which is called regularly by RendererFrameManager::GetMaxNumberOfSavedFrames()
This also allows us to enable reporting for Sierra.

To make this work, GetCurrentPressureLevel is now declared non-const.
There don't appear to be any const MemoryPressureMonitors in the codebase
at the moment, so this should be safe.

BUG=655304

Review-Url: https://codereview.chromium.org/2495003004
Cr-Commit-Position: refs/heads/master@{#434660}