Fix crash after Launcher drag/drop (take 2)

Fix crash after Launcher drag/drop (take 2)

BUG=439055
TEST=In Launcher, drag app to solitary folder on second page

This is a retry of https://codereview.chromium.org/780023002/ ,
which was reverted after commit due to a failed valgrind test.

Committed: https://crrev.com/62e30ca368387d4edbbbf5c721f30881586b2387
Cr-Commit-Position: refs/heads/master@{#309235}

[Greg Levin, 2014-12-19 18:03:08]

glevin@chromium.org changed reviewers:
+ jennyz@chromium.org

[Greg Levin, 2014-12-19 18:03:09]

https://codereview.chromium.org/780023002/ was reverted by
https://codereview.chromium.org/806883004 when valgrind bot failed.

The slow speed of valgrind was somehow altering the behavior of the drag-drop:
in this one case, moving the dragged item over the folder would push the folder
off the second page, and into the now empty last position on the first page,
spoiling the test.  In every other environment, the item hovers over and drops
into the folder, as intended.  Changing the pagination model's transition time
to 0 for this test seems to resolve the issue.

Please have one more look!

[jennyz, 2014-12-19 18:20:15]

lgtm

[jennyz, 2014-12-19 18:23:26]

The CQ bit was checked by jennyz@chromium.org

[commit-bot: I haz the power, 2014-12-19 18:24:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/792883006/1

[commit-bot: I haz the power, 2014-12-19 19:56:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-19 19:56:52]

Patchset 1 (id:??) landed as
https://crrev.com/62e30ca368387d4edbbbf5c721f30881586b2387
Cr-Commit-Position: refs/heads/master@{#309235}

[benwells, 2014-12-22 02:52:39]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/820943003/ by benwells@chromium.org.

The reason for reverting is: The new test is still causing problems. See
http://build.chromium.org/p/chromium.memory.fyi/builders/Chromium%20OS%20%28v....

The test appears to be flaky. It has failed on that bot on 4 of the 6 runs since
this change landed.

BTW I'd add calamity@ to any reviews of dragging in the launcher as he has fixed
many bugs there and is aware of many subtleties in the code..