Revert of [sql] Use memory-mapped I/O for sql::Connection.

Revert of [sql] Use memory-mapped I/O for sql::Connection. (patchset #3 id:40001 of https://codereview.chromium.org/1368533003/ )

Reason for revert:
data race reported by TSan on sql/connection.cc

BUG=537681
Original issue's description:
> [sql] Use memory-mapped I/O for sql::Connection.
>
> sql::Connection::Open*() uses PRAGMA mmap_size to enable SQLite's
> memory-mapped I/O.  Additionally instrument to flush dirty pages from
> the page cache after writes.
>
> [Relands https://crrev.com/9a1948a4d6d4,
> reverted at https://crrev.com/55c3216b15ce ]
>
> BUG=489784, 533682
>
> Committed: https://crrev.com/5c2f4e50d96d8b2ae0ae4a5c7d319e6be3b33bca
> Cr-Commit-Position: refs/heads/master@{#351344}

TBR=rmcilroy@chromium.org,pavely@chromium.org,shess@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=489784, 533682
Committed: https://crrev.com/745e18c523846de216c2f2253beb4e8047f77e57
Cr-Commit-Position: refs/heads/master@{#351596}

[zhaoqin1, 2015-09-30 17:59:51]

Created Revert of [sql] Use memory-mapped I/O for sql::Connection.

[commit-bot: I haz the power, 2015-09-30 18:00:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1375583003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1375583003/1

[commit-bot: I haz the power, 2015-09-30 18:02:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1375583003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1375583003/1

[commit-bot: I haz the power, 2015-09-30 18:04:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-30 18:05:28]

Patchset 1 (id:??) landed as
https://crrev.com/745e18c523846de216c2f2253beb4e8047f77e57
Cr-Commit-Position: refs/heads/master@{#351596}