Finalize thread-safety design for SafeBrowsingDatabase.

Finalize thread-safety design for SafeBrowsingDatabase.

By introducing SafeBrowsingDatabase::DatabaseStateManager:
SafeBrowsingDatabase::ThreadSafeStateManager's NonThreadSafe twin to
manage the DB only state and ensure it's only accessed from the DB's main
thread.

Also turn scoped_ptr<SafeBrowsingStore> into const scoped_ptr<SafeBrowsingStore>:
making it impossible to swap the existing SafeBrowsingStores; thread-safety
being otherwise ensured by SafeBrowsingStore's NonThreadSafe impls.

This only leaves |reset_factory_| behind as an unchecked member, but it's
usage is very restricted and anyone using it further should know to be
careful (added extra documentation to highlight this).

BUG=440517
Committed: https://crrev.com/2d7b2b2d867cd4dfd348fe8fccb235bba842dc1b
Cr-Commit-Position: refs/heads/master@{#315998}

[gab, 2014-12-23 21:27:12]

Patchset #2 (id:20001) has been deleted

[gab, 2014-12-23 21:30:44]

Patchset #1 (id:1) has been deleted

[gab, 2014-12-23 21:40:39]

gab@chromium.org changed reviewers:
+ mattm@chromium.org

[gab, 2014-12-23 21:40:40]

Hey Matt, this is the follow-up to
https://codereview.chromium.org/794273002/diff/40001/chrome/browser/safe_brow...

Thanks!
Gab

[gab, 2015-01-30 12:46:35]

Hey Matt, please take a look and let me know if you still think this is
something we want or whether we can close this and issue 440517.

Thanks,
Gab

[gab, 2015-02-10 13:48:44]

On 2015/01/30 12:46:35, gab wrote:
> Hey Matt, please take a look and let me know if you still think this is
> something we want or whether we can close this and issue 440517.

friendly ping :-)

> 
> Thanks,
> Gab

[mattm, 2015-02-10 21:33:36]

Oops, sorry about that.

lgtm

[gab, 2015-02-12 16:35:54]

New patchsets have been uploaded after l-g-t-m from mattm@chromium.org

[gab, 2015-02-12 16:40:24]

The CQ bit was checked by gab@chromium.org

[commit-bot: I haz the power, 2015-02-12 16:40:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/814993003/80001

[commit-bot: I haz the power, 2015-02-12 16:54:31]

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

[commit-bot: I haz the power, 2015-02-12 16:54:32]

Try jobs failed on following builders:
  linux_chromium_gn_rel on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[gab, 2015-02-12 17:01:33]

The CQ bit was checked by gab@chromium.org

[commit-bot: I haz the power, 2015-02-12 17:01:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/814993003/100001

[commit-bot: I haz the power, 2015-02-12 18:07:50]

Committed patchset #4 (id:100001)

[commit-bot: I haz the power, 2015-02-12 18:08:21]

Patchset 4 (id:??) landed as
https://crrev.com/2d7b2b2d867cd4dfd348fe8fccb235bba842dc1b
Cr-Commit-Position: refs/heads/master@{#315998}