Create MemoryPressure component.

This is the first CL in the refactoring of base/memory/memory_pressure_*. Futher CLs will introduce calculators for each platform, the notification and UMA reporting logic.

BUG=520962

Review URL: https://codereview.chromium.org/1293873002

Cr-Commit-Position: refs/heads/master@{#345081}
20 files changed