This adds ChromeClient methods so core can call the WebViewClient methods for
dragging removing the need for DragClient. It also merges all the code from
DragClient down into core so the drag logic lives in one places. To do this I
also had to move the WebDragOperation.h header into platform so both web/ and
core/ could depend on it.
This also exposed some dead code in web/, like tracking the last mouse position
for dragging, but nothing actually used that value.
BUG=603795
Review URL: https://codereview.chromium.org/1888773003
Cr-Commit-Position: refs/heads/master@{#387639}