Files.app: Store thumbnailUrl from external metadata provider only when it is requested or gotten.

Files.app: Store thumbnailUrl from external metadata provider only when it is requested or gotten.

Storing 'undefined' value when the thumbnail url is not requested will result
in missing thumbnails on FSP volumes.
This CL fixes the issue by storing metadata only when it is requested or gotten
without request.
We should apply the same treatment for all properties, but this CL do it only
for thumbnailUrl to minimize the impact and prepare for merge.
For more context, please see the comments in http://crrev.com/1158463004

BUG=489595, 494910
TEST=Install TED talks extension, switch to grid view, and check thumbnails exist.

Committed: https://crrev.com/60cd0f79fd0f3065c2f5a1f44e19d99a09fd8888
Cr-Commit-Position: refs/heads/master@{#332532}

[fukino, 2015-06-02 05:50:50]

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

[fukino, 2015-06-02 05:50:51]

hirono@, could you take a look? Thanks!

[mtomasz, 2015-06-02 06:13:01]

mtomasz@chromium.org changed reviewers:
+ mtomasz@chromium.org

[mtomasz, 2015-06-02 06:13:02]

Thanks for fixing it! lgtm from me, but please wait for @hirono's lgtm who knows
more about the new metadata cache design.

[hirono, 2015-06-02 17:26:12]

lgtm. Thank you for fixing this!

[fukino, 2015-06-03 01:29:11]

Thanks!

[fukino, 2015-06-03 01:29:19]

The CQ bit was checked by fukino@chromium.org

[commit-bot: I haz the power, 2015-06-03 01:31:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1144393007/1

[commit-bot: I haz the power, 2015-06-03 03:18:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-03 03:20:05]

Patchset 1 (id:??) landed as
https://crrev.com/60cd0f79fd0f3065c2f5a1f44e19d99a09fd8888
Cr-Commit-Position: refs/heads/master@{#332532}