Handle drags ending while being initialized.

Tab drag sessions need mouse capture to function; losing capture
therefore ends the drag. There are, evidently, some cases where
SetCapture() can cause the drag session to lose capture, thereby
ending the drag as it's beginning.

Bug: 964322
Change-Id: I6bcd53f04aefb7461e6c28abf1085df1a012545a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817211
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698687}

chrome/browser/ui/views/tabs/tab_drag_controller.cc

@@ -488,8 +488,14 @@ void TabDragController::Init(TabDragContext* source_context,
 
   // Gestures don't automatically do a capture. We don't allow multiple drags at
   // the same time, so we explicitly capture.
-  if (event_source == EVENT_SOURCE_TOUCH)
+  if (event_source == EVENT_SOURCE_TOUCH) {
+    // Taking capture may cause capture to be lost, ending the drag and
+    // destroying |this|.
+    base::WeakPtr<TabDragController> ref(weak_factory_.GetWeakPtr());
     SetCapture(source_context_);
+    if (!ref)
+      return;
+  }
 
   window_finder_ = std::make_unique<WindowFinder>();
 }