Fix app launcher animation tearing on OSX Mavericks.

Mavericks introduced [NSView setCanDrawSubviewsIntoLayer:] which I
suspect has changed some of the animation compositing pipeline.
Recently, the app launcher has been "tearing" the canvas while dragging
items around quickly.

This change ensures that the common ancestor view of the
NSCollectionView and the transitive layer used for custom drag
animations is layer-backed. This prevents the tearing.

BUG=341769
TEST=Drag items around the OSX app lanucher quickly - icons should
animate smoothly, without tearing.

Review URL: https://codereview.chromium.org/159573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251030 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251030
Files.app: add test for multi-profile desktop movement menu.

BUG=341224

Review URL: https://codereview.chromium.org/145813010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251031 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251031