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This CL fixes an issue where drag and drop wouldn't work on Mac because
the drop data was being cleared by the bookmark manager API's drop
handler before the bookmark manager's handler had handled the drop. This
has been fixed by adding a setTimeout to clearing the data, which was
also in the old bookmark manager, giving the bookmark manager a chance
to deal with the drop.

BUG=716257
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2845243003
Cr-Commit-Position: refs/heads/master@{#468281}