build: Split drag_drop_types.h into its own target

Create //ui/base/dragdrop:types, to allow dependencies on
ui/base/dragdrop/drag_drop_types.h without having to depend on
//ui/base.

This file has only data type definitions and having a separate target
will allow its inclusion in places where it's currently missing. It's
also a precursor CL to move blink::DragOperationsMask there.

Bug: 1093536
Change-Id: I3c1168a918b5d6e59fbce96389f6bd034f6d136a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2770398
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#865870}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index 38f9f4f..bfba53d 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -134,6 +134,7 @@
 
   public_deps = [
     "//ui/base/cursor:cursor_base",
+    "//ui/base/dragdrop:types",
     "//ui/base/dragdrop/mojom:mojom_headers",
     "//ui/base/ime",
     "//ui/compositor",