Revert of base: Replace PurgeAndTruncate with Shrink function.

Revert of base: Replace PurgeAndTruncate with Shrink function. (patchset #5 id:80001 of https://codereview.chromium.org/1018743003/)

Reason for revert:
Suspecting that this might be related to crbug.com/471188.

Original issue's description:
> base: Replace PurgeAndTruncate with Shrink function.
>
> This cleans up the API a bit and allows the code to be unit tested.
>
> This also prevents ftruncate from being used on Android where
> shared memory is backed by Ashmem. Ashmem doesn't support ftruncate.
>
> BUG=466509
> TEST=base_unittests --gtest_filter=DiscardableSharedMemoryTest.Shrink
>
> Committed: https://crrev.com/2d15df9ef3f9b982aef650d0be0be64a1c5d34a5
> Cr-Commit-Position: refs/heads/master@{#322207}

TBR=avi@chromium.org,danakj@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=466509
Committed: https://crrev.com/f2580f56cb770069af82eb10e8e2a58a2f4f72d7
Cr-Commit-Position: refs/heads/master@{#323240}

[reveman, 2015-04-01 15:14:51]

Created Revert of base: Replace PurgeAndTruncate with Shrink function.

[commit-bot: I haz the power, 2015-04-01 15:15:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1057493005/1

[commit-bot: I haz the power, 2015-04-01 15:18:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-01 15:20:15]

Patchset 1 (id:??) landed as
https://crrev.com/f2580f56cb770069af82eb10e8e2a58a2f4f72d7
Cr-Commit-Position: refs/heads/master@{#323240}