Reset DragState::m_dragSrc when mouse move doesn't meet drag threshold.

third_party/WebKit/Source/core/input/MouseEventManager.cpp

 // Copyright 2016 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 "core/input/MouseEventManager.h"
 
 #include "core/clipboard/DataObject.h"
 #include "core/clipboard/DataTransfer.h"
 #include "core/dom/Element.h"
 #include "core/dom/ElementTraversal.h"
@@ skipping 770 matching lines @@
                 .mouseDownMayStartSelect() &&
            !m_mouseDownMayStartAutoscroll;
   }
 
   // We are starting a text/image/url drag, so the cursor should be an arrow
   // FIXME <rdar://7577595>: Custom cursors aren't supported during drag and
   // drop (default to pointer).
   m_frame->view()->setCursor(pointerCursor());
 
   if (initiator == DragInitiator::Mouse &&
-      !dragThresholdExceeded(event.event().position()))
+      !dragThresholdExceeded(event.event().position())) {
+    dragState().m_dragSrc = nullptr;
     return true;
+  }
 
   // Once we're past the drag threshold, we don't want to treat this gesture as
   // a click.
   invalidateClick();
 
   if (!tryStartDrag(event)) {
     // Something failed to start the drag, clean up.
     clearDragDataTransfer();
     dragState().m_dragSrc = nullptr;
   }
@@ skipping 183 matching lines @@
 
 void MouseEventManager::setClickCount(int clickCount) {
   m_clickCount = clickCount;
 }
 
 bool MouseEventManager::mouseDownMayStartDrag() {
   return m_mouseDownMayStartDrag;
 }
 
 }  // namespace blink