Files.app: Add createSnapshot, clone, hasFreashCache methods to metadata models.

Files.app: Add createSnapshot, clone, hasFreashCache methods to metadata models.

The CL:
 * Adds createSnapshot and hasFreshCache methods to MetadataCacheSet
 * Adds clone and hasFreshCache mehtods to MetadataCacheItem

The createSnapshot and clone are the methods to generage deep copy of the model.
The hasFreshCache is the method to check the cache contains all required caches.

BUG=410766
TEST=FileManagerJsTest.MetadataCacheSet,FileManagerJsTest.MetadataCacheItem

Committed: https://crrev.com/c69d6c6efff672ca757217d73746b9c8f87cb4d8
Cr-Commit-Position: refs/heads/master@{#314259}

[hirono, 2015-02-03 00:31:12]

hirono@chromium.org changed reviewers:
+ yawano@chromium.org

[hirono, 2015-02-03 00:31:13]

PTAL, thanks!

[yawano, 2015-02-03 01:26:08]

lgtm.

[hirono, 2015-02-03 01:29:45]

The CQ bit was checked by hirono@chromium.org

[hirono, 2015-02-03 01:30:09]

On 2015/02/03 01:26:08, yawano wrote:
> lgtm.

Thanks!

[commit-bot: I haz the power, 2015-02-03 01:30:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/898533003/20001

[commit-bot: I haz the power, 2015-02-03 02:31:05]

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

[commit-bot: I haz the power, 2015-02-03 02:31:06]

Try jobs failed on following builders:
  win8_chromium_rel on tryserver.chromium.win (None)

[hirono, 2015-02-03 03:21:43]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2015-02-03 03:22:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/898533003/20001

[commit-bot: I haz the power, 2015-02-03 03:26:36]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-03 03:28:16]

Patchset 2 (id:??) landed as
https://crrev.com/c69d6c6efff672ca757217d73746b9c8f87cb4d8
Cr-Commit-Position: refs/heads/master@{#314259}