Support dragging texts into Android WebView.

content/public/browser/render_view_host.h

Index: content/public/browser/render_view_host.h
diff --git a/content/public/browser/render_view_host.h b/content/public/browser/render_view_host.h
index 1a4d1646d708a80fc6c84e0c3b7e821212ca1f98..5ed35ad303786a11b15f070706d36b2963d85d1e 100644
--- a/content/public/browser/render_view_host.h
+++ b/content/public/browser/render_view_host.h
@@ -145,6 +145,12 @@ class CONTENT_EXPORT RenderViewHost : public IPC::Sender {
   virtual void DragTargetDrop(const gfx::Point& client_pt,
                               const gfx::Point& screen_pt,
                               int key_modifiers) = 0;
+#if defined(OS_ANDROID)
+  virtual void DragTargetDropWithData(const DropData& drop_data,

[aelias_OOO_until_Jul13, 2016/02/25 20:58:00]

Why do you need to add a new platform-specific method?  This has close to the
same arguments to DragTargetDragEnter, what is the meaning/value-added of this
new method?

[hush (inactive), 2016/02/27 01:46:13]

See CL: https://codereview.chromium.org/1723763002/
On Android, we don't have the text being dragged until ACTION_DROP. So the
DropData that's passed during DragTargetDragEnter does not contain the real text
being dragged. We have to pass another DropData during ACTION_DROP, and that's
what this function does.
On other platforms (aura), we have access to the dragged when the drag starts,
so there is no need to specify the drop data again during 'drop' action.

[aelias_OOO_until_Jul13, 2016/02/27 01:54:41]

It doesn't seem great that we're passing in fake/empty data in
DragTargetDragEnter, that seems misleading.  Can you remove the DropData from
DragTargetDragEnter and refactor all platforms to use your new DropData IPC?

[hush (inactive), 2016/02/29 18:27:41]

I can do that. But this would bloat this CL significantly though. I would do it
in another CL that sits between https://codereview.chromium.org/1723763002/ and
this CL.

[hush (inactive), 2016/03/01 18:54:28]

Hi Alex, 
I did some investigation yesterday and it turns out I can't remove the dropdata
from DragEnter on all platforms as that would break the drag and drop spec. 
https://www.w3.org/TR/2010/WD-html5-20101019/dnd.html#the-dragevent-and-datat...

Javascript API requires that "dataTransfer" object which contains the dragged
data is available when the drag starts, and when it updates its location. 

That is to say, Android's drag and drop is breaking the HTML spec inevitably. We
might just have to live with that, passing an empty dummy data when the drag
starts. At least we're indicating what the type of the data is. 

We could ask Android to make the data available when the drag starts (if it is
possible), but that's for another Android release.

+                                      const gfx::Point& client_pt,
+                                      const gfx::Point& screen_pt,
+                                      int key_modifiers) = 0;
+#endif
 
   // Instructs the RenderView to automatically resize and send back updates
   // for the new size.