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

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

Index: third_party/WebKit/Source/core/input/MouseEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/MouseEventManager.cpp b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
index d84acf00280e6d281215ce4200b0726d8bd3121a..29026f3ba4673a821acaa61dbc3369bbbdd318a2 100644
--- a/third_party/WebKit/Source/core/input/MouseEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
@@ -788,8 +788,10 @@ bool MouseEventManager::handleDrag(const MouseEventWithHitTestResults& event,
   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.