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

[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}

[Scott Hess - ex-Googler, 2015-09-24 03:12:02]

Disable mmap test under mojo.

[Scott Hess - ex-Googler, 2015-09-24 03:12:53]

Fix uninit vars in Connection.

[Scott Hess - ex-Googler, 2015-09-24 03:13:32]

https://codereview.chromium.org/1349863003/, slipstream
https://codereview.chromium.org/1355403004/ to fix mojo test, slipstream
https://codereview.chromium.org/1368493002/ to fix valgrind complaint.

[Scott Hess - ex-Googler, 2015-09-24 03:14:02]

On 2015/09/24 03:13:32, Scott Hess wrote:
> https://codereview.chromium.org/1349863003/, slipstream
> https://codereview.chromium.org/1355403004/ to fix mojo test, slipstream
> https://codereview.chromium.org/1368493002/ to fix valgrind complaint.

First CL was reverted at https://codereview.chromium.org/1365783002

[Scott Hess - ex-Googler, 2015-09-24 03:15:58]

The CQ bit was checked by shess@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-24 03:17:10]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1368533003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1368533003/40001

[commit-bot: I haz the power, 2015-09-24 04:53:53]

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

[commit-bot: I haz the power, 2015-09-24 04:53:53]

Dry run: This issue passed the CQ dry run.

[Scott Hess - ex-Googler, 2015-09-28 19:26:15]

shess@chromium.org changed reviewers:
+ pavely@chromium.org, rmcilroy@chromium.org

[Scott Hess - ex-Googler, 2015-09-28 19:26:16]

https://codereview.chromium.org/1349863003/ got reverted, for one
hard-to-predict reason (Mojo does some hackery WRT SQLite) and one
should-have-predicted reason (I was looking mostly for REALLY scary things and
forgot to init the new instance variables).  I manually ran sql_unittests
through valgrind, and ps#3 addressed the items thrown up by the previous
landing.

All of the bits for this were already approved, so passing back to the main
reviewer (rmcilroy) and the OWNER for sync (pavely) for rubber stamp or
re-review as they wish.

Thanks.

[rmcilroy, 2015-09-28 19:53:46]

lgtm

[pavely, 2015-09-28 20:40:45]

sync/* lgtm

[Scott Hess - ex-Googler, 2015-09-29 16:03:47]

The CQ bit was checked by shess@chromium.org

[commit-bot: I haz the power, 2015-09-29 16:04:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1368533003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1368533003/40001

[commit-bot: I haz the power, 2015-09-29 17:21:31]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-09-29 17:22:18]

Patchset 3 (id:??) landed as
https://crrev.com/5c2f4e50d96d8b2ae0ae4a5c7d319e6be3b33bca
Cr-Commit-Position: refs/heads/master@{#351344}

[zhaoqin1, 2015-09-30 17:59:50]

A revert of this CL (patchset #3 id:40001) has been created in
https://codereview.chromium.org/1375583003/ by zhaoqin@chromium.org.

The reason for reverting is: data race reported by TSan on sql/connection.cc

BUG=537681.