Add mojo support for content::DropData

Cut-and-paste IPC traits for content::DropData from its non-public home
in drag_traits.h to the public common_param_traits_macros.h.

Add mojo definitions and typemaps.

Bug: 898608
Change-Id: I6f9f3d5f2ff479a84dae20deacc34513611929e1
Reviewed-on: https://chromium-review.googlesource.com/c/1397421
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622300}
7 files changed