Fix crash after Launcher drag/drop

Fix crash after Launcher drag/drop

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

In the case described, AppsGridView::EndDrag()'s call to MoveItemToFolder() makes a recursive call to AppsGridView::EndDrag(), which deletes drag_view_.  Hence the dereferencing NULL when we return to the top-level call.

Committed: https://crrev.com/286e841626dea4e579b5ddfc86fe8e3aec8bcaa9
Cr-Commit-Position: refs/heads/master@{#308404}

[Greg Levin, 2014-12-04 18:44:24]

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

[Greg Levin, 2014-12-04 18:49:01]

Please have a look!

As a possible alternative fix, I could add
  if (!drag_view_)
    return;
between "MoveItemToFolder()" and "if (drag_and_drop_host_)".  Everything after
this seems to be drag/drop cleanup, which has (presumably) already been done in
the recursive call to "MoveItemToFolder()".  I tried both versions of the fix in
this circumstance, and both seemed to work fine.  Thoughts on which is better?

[jennyz, 2014-12-04 19:26:59]

The fix looks fine to me, is it possible to add a test case for this?

[Greg Levin, 2014-12-11 23:36:03]

> is it possible to add a test case for this?

Yes, and no.  I added a unit test for this situation, but even without my
changes to apps_grid_view.cc, the test passes :-(  The AppsGridViewTest
environment doesn't include a drag_and_drop_host_, so the code block where the
SegFault occurred is not entered.  As per our discussion, getting the test to
detect this bug isn't worth the extra plumbing.  I'm including the unit test
nonetheless, since it's an interesting edge case, so please have a look at it!

[jennyz, 2014-12-13 00:23:53]

lgtm

[jennyz, 2014-12-15 18:16:33]

The CQ bit was checked by jennyz@chromium.org

[commit-bot: I haz the power, 2014-12-15 18:17:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/780023002/20001

[commit-bot: I haz the power, 2014-12-15 20:23:32]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-15 20:24:20]

Patchset 2 (id:??) landed as
https://crrev.com/286e841626dea4e579b5ddfc86fe8e3aec8bcaa9
Cr-Commit-Position: refs/heads/master@{#308404}

[Greg Levin, 2014-12-19 17:02:49]

Patchset #3 (id:40001) has been deleted

[Greg Levin, 2014-12-19 17:17:00]

MouseDragItemIntoLonelyFolder unit test was failing under valgrind.  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.  Changing the pagination model's transition time to 0 for this test
seems to resolve the issue.