GitLab

When we implemented MemoryCoordinatorClients in net/, there was only one
callback called OnMemoryStateChange(). Recently we added another
callback called OnPurgeMemory() to handle "existing allocations" and
"future allocations" separately (See [1] for the motivation). This CL
replaces OnMemoryStateChange() with OnPurgeMemory() because all clients
in net/ actually free up memory.

Note: MemoryCoordinatorClient and MemoryPressureListener won't be
enabled at the same time. This means that either OnMemoryPressure() or
OnPurgeMemory() can be called.

[1] https://groups.google.com/a/chromium.org/forum/?utm_medium=email&utm_source=footer#!msg/project-trim/s96xSirL2Hs/18uq1zfHEgAJ

BUG=684287,617492

Review-Url: https://codereview.chromium.org/2679903002
Cr-Commit-Position: refs/heads/master@{#449178}