[sql] Validate database files before enabling memory-mapping.

[sql] Validate database files before enabling memory-mapping.

With regular I/O, filesystem corruption would cause SQLite to return
SQLITE_IOERR codes.  With memory-mapping, filesystem errors can cause a
crash when accessed.  There are databases with filesystem corruption,
this CL only enables memory-mapped I/O for parts of the database which
have been successfully read at some point using regular I/O.

[Relands https://codereview.chromium.org/1426743006 which was reverted at
 https://codereview.chromium.org/1432953002 .  CL is identical, fix to
 sql::Connection was landed at https://codereview.chromium.org/1426743006 .]

BUG=537742
TBR=rmcilroy@chromium.org, isherman+reviewer@chromium.org

Committed: https://crrev.com/d90aeea87619c019f8b4be171b65476748868ece
Cr-Commit-Position: refs/heads/master@{#359478}

[Scott Hess - ex-Googler, 2015-11-13 01:05:36]

The CQ bit was checked by shess@chromium.org

[Scott Hess - ex-Googler, 2015-11-13 01:08:21]

TBR because this CL is identical to the earlier:
   https://codereview.chromium.org/1426743006

Sheriffs (or bystanders), the previous CL broke some ChromiumOS tests, see:
   https://codereview.chromium.org/1432953002

I have landed a different CL:
   https://codereview.chromium.org/1434173003
which I have verified allows this CL to work with the ChromiumOS build's tests. 
That CL seemed to take the tests from "not working" to "working", so I'm
assuming I got the build config right, but ...

[commit-bot: I haz the power, 2015-11-13 01:14:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1442753004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1442753004/1

[commit-bot: I haz the power, 2015-11-13 02:24:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-13 02:25:38]

Patchset 1 (id:??) landed as
https://crrev.com/d90aeea87619c019f8b4be171b65476748868ece
Cr-Commit-Position: refs/heads/master@{#359478}