This has to be merged back to m53 because it is enabled by default in m53.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/29ec4189153a39f8730991f727f58aa4dae7fe35

commit 29ec4189153a39f8730991f727f58aa4dae7fe35
Author: hush <hush@chromium.org>
Date: Sat Jul 09 00:03:17 2016

Gate Android drag and drop feature with a flag.

We want to fully test it before enabling by default.

BUG= 626773 

Review-Url: https://codereview.chromium.org/2133303002
Cr-Commit-Position: refs/heads/master@{#404540}

[modify] https://crrev.com/29ec4189153a39f8730991f727f58aa4dae7fe35/content/browser/android/content_view_core_impl.cc
[modify] https://crrev.com/29ec4189153a39f8730991f727f58aa4dae7fe35/content/browser/android/content_view_core_impl.h
[modify] https://crrev.com/29ec4189153a39f8730991f727f58aa4dae7fe35/content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java

Can I merge this CL to m53?

Your change meets the bar and is auto-approved for M53 (branch: 2785)

Sorry actually no need to get this to m53.
The CL that really enabled drag and drop only went to m54.

commit 445de0ecb75960ee4060dfcfaf1a86e5632e4d0a
Author: hush <hush@chromium.org>
Date:   Thu Jun 30 23:52:45 2016 -0700

    Support dragging texts into Android WebView.
    
    WebView gets callback 'onDragEvent' when something is dragged into the WebView.
    This CL extracts the texts (if any) and location from the DragEvent and creates
    DropData which is Chromium's internal representation of dragged data.
    
    The event plumbing path is WebView#onDragEvent --> AwContents#onDragEvent -->
    ContentViewCore --> WebContentsViewAndroid.
    
    This CL also uses a new blink::WebView API that performs dropping with DropData,
    because on Android, the DropData is only available when the drop is performed,
    not when the drag is started.
    
    BUG= 584789 
    
    Review-Url: https://codereview.chromium.org/1728193002
    Cr-Commit-Position: refs/heads/master@{#403417}