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This is a step towards unifying the various ways to "notify"
MemoryPressureListeners.

This also provides "GetCurrentPressureLevel" to a more general-purpose
place so we'll be able to use it on other platforms.
(Especially Windows, so we can use it in renderer_frame_manager).

See:
https://docs.google.com/a/chromium.org/document/d/1z3SR3JfKOwVaZxrsfHLOrPjo4-IVtnijaX86LKYPEQs/edit#
for more on the rationale and design, and
https://codereview.chromium.org/1045433002/
for what this might look like for ChromeOS's memory
monitor.

BUG=463546

Review URL: https://codereview.chromium.org/1048983006

Cr-Commit-Position: refs/heads/master@{#324086}