MacPWAs drag-drop: Create a structure for drag parameters

Create a DraggingInfo structure with all parameters that are needed for
WebDragDest's  draggingEntered, draggingUpdated, and
performDragOperation methods.

This structure will be mojo-ified and then passed from the
WebContentsViewCocoa (in the PWA process) to the WebDragDest (in the
browser process).

Bug: 898608
Change-Id: I2f88c081599d2e89c5a4a3117fb8b867968b18af
Reviewed-on: https://chromium-review.googlesource.com/c/1407969
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622319}
4 files changed