Files.app: Add guard closure for selectedAsyncData.

Files.app: Add guard closure for selectedAsyncData.

The FileTransferController#selectedAsyncData is cache of data that cannot be obtained synchronously.
This is needed for assigning the data to DataTransfer in the event handler.
The data can be null if the event is dispatched before the data is obtained.

BUG=431650
TEST=None

Committed: https://crrev.com/5bb64a5b8ff45d1326552f70f687f378a76af97f
Cr-Commit-Position: refs/heads/master@{#303444}

[hirono, 2014-11-10 09:22:28]

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

[hirono, 2014-11-10 09:22:54]

PTAL the CL? Thanks!

[fukino, 2014-11-10 09:48:18]

On 2014/11/10 09:22:54, hirono wrote:
> PTAL the CL? Thanks!

lgtm

[hirono, 2014-11-10 09:49:39]

The CQ bit was checked by hirono@chromium.org

[commit-bot: I haz the power, 2014-11-10 09:49:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/704333003/1

[commit-bot: I haz the power, 2014-11-10 10:34:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-10 10:35:22]

Patchset 1 (id:??) landed as
https://crrev.com/5bb64a5b8ff45d1326552f70f687f378a76af97f
Cr-Commit-Position: refs/heads/master@{#303444}