GitLab

Created a Drag & Drop client on Mac that bridges between the Views
and native OSX's drag and drop. This approach is based on the Aura
approach.

BUG=599585
TEST= Run the view unit test: DragDropClientMacTest

Review-Url: https://codereview.chromium.org/1964283002
Cr-Commit-Position: refs/heads/master@{#397801}