Re-land: base: Fix DiscardableSharedMemory::mapped_size().

Re-land: base: Fix DiscardableSharedMemory::mapped_size().

Set |mapped_size_| to 0 after unmapping memory.

BUG=429416
TEST=base_unittests --gtest_filter=DiscardableSharedMemoryTest.MappedSize

Committed: https://crrev.com/6e3fb363a511053170e5b019c733dd75ad406100
Cr-Commit-Position: refs/heads/master@{#311051}

[reveman, 2015-01-08 19:44:42]

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

[reveman, 2015-01-08 19:44:42]

This is what made https://codereview.chromium.org/793443003/ cause crashes and
get reverted. Please take a look.

[danakj, 2015-01-08 19:45:39]

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

[danakj, 2015-01-08 19:45:40]

Can you run this by Avi also?

[danakj, 2015-01-08 19:45:54]

On 2015/01/08 19:44:42, reveman wrote:
> This is what made https://codereview.chromium.org/793443003/ cause crashes and
> get reverted. Please take a look.

Should we be unit testing this?

[reveman, 2015-01-08 19:57:40]

added a unit test. ptal.

[danakj, 2015-01-08 20:02:08]

Thanks, LGTM % avi

[Avi (use Gerrit), 2015-01-08 21:18:43]

On 2015/01/08 20:02:08, danakj wrote:
> Thanks, LGTM % avi

S LGTM

[reveman, 2015-01-08 21:40:05]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2015-01-08 21:41:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/842003002/20001

[commit-bot: I haz the power, 2015-01-08 23:26:50]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-08 23:27:47]

Patchset 2 (id:??) landed as
https://crrev.com/95bc5b1779dcf8e4a9e37ef600c0ea76293307e3
Cr-Commit-Position: refs/heads/master@{#310625}

[pennymac, 2015-01-10 00:24:02]

A revert of this CL (patchset #2 id:20001) has been created in
https://codereview.chromium.org/810433003/ by pennymac@google.com.

The reason for reverting is: Reverting this 2nd CL that touches
discardable_shared_memory.

High levels of Windows crashes resulted from something in one of these two CLs
(https://codereview.chromium.org/793443003 is the other).

Ref:
https://crash.corp.google.com/browse?q=product.name%3D%27Chrome%27%20AND%20pr....

[reveman, 2015-01-12 15:21:11]

On 2015/01/10 00:24:02, pennymac wrote:
> A revert of this CL (patchset #2 id:20001) has been created in
> https://codereview.chromium.org/810433003/ by mailto:pennymac@google.com.
> 
> The reason for reverting is: Reverting this 2nd CL that touches
> discardable_shared_memory.
> 
> High levels of Windows crashes resulted from something in one of these two CLs
> (https://codereview.chromium.org/793443003 is the other).
> 
> Ref:
>
https://crash.corp.google.com/browse?q=product.name%3D%27Chrome%27%20AND%20pr....

Re-landing as this change is not the reason for these crashes.

[reveman, 2015-01-12 15:21:22]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2015-01-12 15:22:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/842003002/20001

[commit-bot: I haz the power, 2015-01-12 15:32:44]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-12 15:33:37]

Patchset 2 (id:??) landed as
https://crrev.com/6e3fb363a511053170e5b019c733dd75ad406100
Cr-Commit-Position: refs/heads/master@{#311051}