base: Improve DiscardableSharedMemory support for ashmem.
Locking can now succeed even when the range of pages has been
purged. This avoids unnecessary IPC as it reduces the need to
create new DiscardableSharedMemory instances when backed by ashmem.
BUG=429416
TEST=base_unittests --gtest_filter=DiscardableSharedMemoryTest*
Review URL: https://codereview.chromium.org/871043003
Cr-Commit-Position: refs/heads/master@{#312992}
CrOS-Libchrome-Original-Commit: 876f863615f341236004e0c3cd47125311e4c2d4
4 files changed