base: Fix DiscardableSharedMemory locking problem.

base: Fix DiscardableSharedMemory locking problem.

Null last_known_usage should not cause
DiscardableSharedMemory::Lock to fail when some pages are
already locked.

TEST=base_unittests --gtest_filter=DiscardableSharedMemoryTest.LockAndUnlockRange
BUG=429415
Committed: https://crrev.com/5a854d30eb6cae7ffa6a725d5257f027c153b7c0
Cr-Commit-Position: refs/heads/master@{#316706}

[reveman, 2015-02-17 21:20:12]

reveman@chromium.org changed reviewers:
+ avi@chromium.org

[Avi (use Gerrit), 2015-02-17 21:21:40]

LGTM

[reveman, 2015-02-17 21:23:15]

reveman@chromium.org changed reviewers:
+ danakj@chromium.org

[reveman, 2015-02-17 21:23:15]

+danakj for base/

[danakj, 2015-02-17 21:42:57]

https://codereview.chromium.org/931303002/diff/1/base/memory/discardable_shar...
File base/memory/discardable_shared_memory_unittest.cc (left):

https://codereview.chromium.org/931303002/diff/1/base/memory/discardable_shar...
base/memory/discardable_shared_memory_unittest.cc:238:
EXPECT_EQ(DiscardableSharedMemory::FAILED, lock_rv);
Maybe change this to what memory1.Lock should return now with a comment
explaining?

[reveman, 2015-02-17 22:09:43]

https://codereview.chromium.org/931303002/diff/1/base/memory/discardable_shar...
File base/memory/discardable_shared_memory_unittest.cc (left):

https://codereview.chromium.org/931303002/diff/1/base/memory/discardable_shar...
base/memory/discardable_shared_memory_unittest.cc:238:
EXPECT_EQ(DiscardableSharedMemory::FAILED, lock_rv);
On 2015/02/17 at 21:42:57, danakj wrote:
> Maybe change this to what memory1.Lock should return now with a comment
explaining?

Locking/unlocking two different mappings of the memory is not supported. It was
a mistake to have this check here. This will correctly fail in a DCHECK now this
bug has been fixed.

[danakj, 2015-02-17 22:41:59]

OK Thanks LGTM

[reveman, 2015-02-17 22:47:25]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2015-02-17 22:48:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/931303002/1

[commit-bot: I haz the power, 2015-02-18 00:06:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-18 00:07:36]

Patchset 1 (id:??) landed as
https://crrev.com/5a854d30eb6cae7ffa6a725d5257f027c153b7c0
Cr-Commit-Position: refs/heads/master@{#316706}