GitLab

Pull out the extensions code drag-and-drop handler in order to reuse it
in the MD extensions page. In the process, closure compiler DragWrapper
and create a real interface for the DragWrapperHandler, which previously
was only specified in comments. Update drag wrapper handler implementors
with @override.

BUG=529395
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2164693002
Cr-Commit-Position: refs/heads/master@{#407242}