Route OnDragEvent through ViewAndroid tree

ui/events/android/drag_event_android.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/events/android/drag_event_android.h"
+
+#include "ui/events/event_utils.h"
+#include "ui/gfx/geometry/point_f.h"
+
+namespace ui {
+
+DragEventAndroid::DragEventAndroid(
+    int action,
+    const gfx::PointF& location,
+    const gfx::PointF& screen_location,
+    const std::vector<base::string16>& mime_types,
+    const base::string16& content)
+    : LocatedEvent(ET_DROP_TARGET_EVENT,
+                   location,
+                   screen_location,
+                   EventTimeForNow(),
+                   0),
+      action_(action),
+      mime_types_(mime_types),
+      content_(content) {}
+
+std::unique_ptr<DragEventAndroid> DragEventAndroid::CreateFor(
+    const gfx::Point& location) const {
+  gfx::PointF new_location(location);
+  gfx::PointF new_screen_location =
+      new_location + (root_location_f() - location_f());

[aelias_OOO_until_Jul13, 2017/05/24 22:25:40]

Terminology and calculation seems weird here.  "screen" position typically
refers to location from the (0,0) coordinate of the physical screen (think
especially of floating window UIs).  root_location_f doesn't seem to refer to
that root, based on the comment for ui/events/event.h: "|location_| multiplied
by an optional transformation matrix for rotations, animations and skews.".  And
this isn't a pattern I see elsewhere.  So maybe just set this to be equal to
new_location?

[Jinsuk Kim, 2017/05/25 03:18:19]

Agreed that storing screen location to root location is misleading. Let me store
the position directly in the class since the parent class doesn't provide a
place for it. Then the parent class is useful for |location_| only, not enough
reason to inherit it. Removed the inheritance.

|screen_location_| still needs adjusting to have it point to the new location
relative to the screen coordinate. I'm still doing the conversion.  

+  return std::unique_ptr<DragEventAndroid>(new DragEventAndroid(
+      action_, new_location, new_screen_location, mime_types_, content_));
+}
+
+}  // namespace ui