[fsp] Move thumbnail logic to Drive specific C++ code.

[fsp] Move thumbnail logic to Drive specific C++ code.

MetadataCache is soon going to be generalized from Drive to also FSP file
systems. So, in the Files app layer, we should not have Drive specific code.

TEST=Tested manually.
BUG=408017
Committed: https://crrev.com/ca54e1562467d112b8e496dc7dfbcaaf63567367
Cr-Commit-Position: refs/heads/master@{#292829}

[mtomasz, 2014-09-01 02:56:44]

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

[mtomasz, 2014-09-01 02:56:44]

@hirono: PTAL at this tiny patch. Thanks.

[hirono, 2014-09-01 03:25:23]

lgtm, thanks!

[mtomasz, 2014-09-01 03:25:44]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-09-01 03:26:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mtomasz@chromium.org/517213002/20001

[commit-bot: I haz the power, 2014-09-01 03:50:16]

Committed patchset #2 (id:20001) as 6423dfc98b2c7765464dd922729a1f536e70e9a6

[commit-bot: I haz the power, 2014-09-10 03:15:27]

Patchset 2 (id:??) landed as
https://crrev.com/ca54e1562467d112b8e496dc7dfbcaaf63567367
Cr-Commit-Position: refs/heads/master@{#292829}