Add a developerPrivate API function to install a file that was dragged
and dropped on the calling web contents, and add tests for the same.
This will allow us to move the MD extensions page away from using
chrome.send for this functionality.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id2b4f5417a94c330c516d4eb1432c59c89c31e32
Reviewed-on: https://chromium-review.googlesource.com/981119Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547776}