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Refactor the DiscardableSharedMemory implementation to put platform-
native discardable page management in separate LockPages/UnlockPages
functions.

This isolates the platform-specific calls from internal details of the
DiscardableSharedMemory implementation, in particular the need to allow
for the space occupied by the SharedState structure.

Also corrects "lock page" platform API errors to return FAILED rather
than PURGED result, and adds some DCHECKing for "unlock page" API
errors.

Bug: 716205
Change-Id: I572d65a0304816305053669ec81c78a3461d8325
Reviewed-on: https://chromium-review.googlesource.com/574901
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488027}