Add type annotations to metadata_dispatcher.js.

Add type annotations to metadata_dispatcher.js.

BUG=444510
TEST=GYP_GENERATORS=ninja tools/gyp/gyp --depth . ui/file_manager/file_manager/foreground/js/metadata/compiled_resources.gyp && ninja -C out/Default

Committed: https://crrev.com/58af8f4de35388970be49fe5fa16c24610d8e813
Cr-Commit-Position: refs/heads/master@{#309625}

[yawano, 2014-12-24 08:29:41]

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

[yawano, 2014-12-24 08:29:42]

PTAL. Thank you!

[fukino, 2014-12-24 08:52:50]

On 2014/12/24 08:29:42, yawano wrote:
> PTAL. Thank you!

lgtm.

[yawano, 2014-12-24 08:53:26]

On 2014/12/24 08:52:50, fukino wrote:
> On 2014/12/24 08:29:42, yawano wrote:
> > PTAL. Thank you!
> 
> lgtm.

Thank you!

[yawano, 2014-12-24 08:53:29]

The CQ bit was checked by yawano@chromium.org

[commit-bot: I haz the power, 2014-12-24 08:54:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/824003004/1

[commit-bot: I haz the power, 2014-12-24 09:33:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-24 09:34:21]

Patchset 1 (id:??) landed as
https://crrev.com/58af8f4de35388970be49fe5fa16c24610d8e813
Cr-Commit-Position: refs/heads/master@{#309625}