Fix a drag-and-drop bug in the bookmark manager on Chrome OS.
Fix a drag-and-drop bug in the bookmark manager on Chrome OS.

Before the fix, drag-and-drop in the bookmark manager on Chrome OS
resulted in a copy rather than move. The drag-and-drop logic is shared
with GTK and Views, but there was a subtle difference between the two
toolkits, that caused the bug.

BUG=chromium-os:7533
TEST=confirmed that dnd in the bookmark manager worked on Linux and Chrome OS

Review URL: http://codereview.chromium.org/3743002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@62524 0039d316-1c4b-4281-b951-d872f2087c98

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