Revert of base: Fix DiscardableSharedMemory::mapped_size().

Revert of base: Fix DiscardableSharedMemory::mapped_size(). (patchset #2 id:20001 of https://codereview.chromium.org/842003002/)

Reason for revert:
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%20product.version%3D%2741.0.2271.0%27%20AND%20custom_data.ChromeCrashProto.ptype%3D%27renderer%27

Original issue's description:
> 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/95bc5b1779dcf8e4a9e37ef600c0ea76293307e3
> Cr-Commit-Position: refs/heads/master@{#310625}

TBR=danakj@chromium.org,avi@chromium.org,reveman@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=429416
Committed: https://crrev.com/86a0ec0ecfbdb49472e73fa365cf41372732e359
Cr-Commit-Position: refs/heads/master@{#310916}

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

Created Revert of base: Fix DiscardableSharedMemory::mapped_size().

[commit-bot: I haz the power, 2015-01-10 00:25:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/810433003/1

[commit-bot: I haz the power, 2015-01-10 00:25:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-01-10 00:25:17]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[danakj, 2015-01-10 00:28:24]

FWIW I don't think you need this. If it's turned off, this shouldn't matter.

[danakj, 2015-01-10 00:28:56]

On 2015/01/10 00:28:24, danakj wrote:
> FWIW I don't think you need this. If it's turned off, this shouldn't matter.

But maybe for just in case? heh. LGTM

[danakj, 2015-01-10 00:28:59]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2015-01-10 00:30:45]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/810433003/1

[commit-bot: I haz the power, 2015-01-10 00:32:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-10 00:33:58]

Patchset 1 (id:??) landed as
https://crrev.com/86a0ec0ecfbdb49472e73fa365cf41372732e359
Cr-Commit-Position: refs/heads/master@{#310916}