Change ImageStore to use SequenceChecker instead of ThreadChecker.

Change ImageStore to use SequenceChecker instead of ThreadChecker.
    
We will be using ImageStore with
BrowserThread::PostBlockingPoolSequencedTask which doesn't always
guarantee same thread execution.
    
BUG=368034
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270801

[sky, 2014-05-13 22:15:03]

I think ImageStore should only be created and used on a single thread. That way
there are no issues like this.

[Kibeom Kim (inactive), 2014-05-15 12:52:10]

changed to use SequenceChecker, ptal.

[sky, 2014-05-15 16:44:53]

LGTM

[Kibeom Kim (inactive), 2014-05-15 16:47:55]

The CQ bit was checked by kkimlabs@chromium.org

[commit-bot: I haz the power, 2014-05-15 16:50:09]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kkimlabs@chromium.org/288573003/40001

[commit-bot: I haz the power, 2014-05-15 18:34:42]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  ios_rel_device on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/ios_rel_device/builds...)
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-15 20:35:50]

Change committed as 270801