Handle the case when the last selection is not set yet.

Handle the case when the last selection is not set yet.

When a user starts to drag an item before they haven't selected any item yet,
FileTable.lastSelection_ can be undefined and this ends up an incorrect drag image.
The existence of lastSelection_ should be checked.

BUG=409514
TEST=manually

Committed: https://crrev.com/60a533c07f2e01cb06e17d829a7972fa9a1b2984
Cr-Commit-Position: refs/heads/master@{#292845}

[fukino, 2014-09-01 06:36:00]

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

[fukino, 2014-09-01 06:36:17]

@hirono, PTAL the CL? Thanks!

[hirono, 2014-09-01 06:42:12]

lgtm!

[fukino, 2014-09-01 06:43:27]

The CQ bit was checked by fukino@chromium.org

[commit-bot: I haz the power, 2014-09-01 06:44:30]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/fukino@chromium.org/532453002/1

[commit-bot: I haz the power, 2014-09-01 08:12:09]

Committed patchset #1 (id:1) as a8f3e814e770c0044298f4a9af2e3b4ea6820c62

[commit-bot: I haz the power, 2014-09-10 03:15:56]

Patchset 1 (id:??) landed as
https://crrev.com/60a533c07f2e01cb06e17d829a7972fa9a1b2984
Cr-Commit-Position: refs/heads/master@{#292845}