Check document availiblity after 'selectStart' event is dispatched

'selectStart' event in UpdateSelectionForMouseDrag can affect the document.
So we should check the DocumentAvailiblity after the 'selectStart' event.
We should also check the cached position validity after
the event is dispatched.

Bug: 751817
Change-Id: Ie024fd9464771865d6611da10f69fb5df43754ce
Reviewed-on: https://chromium-review.googlesource.com/713074
Commit-Queue: Tanvir Rizvi <tanvir.rizvi@samsung.com>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509348}

third_party/WebKit/Source/core/editing/SelectionController.cpp

@@ -476,9 +476,12 @@ void SelectionController::UpdateSelectionForMouseDrag(
       DispatchSelectStart(target) != DispatchEventResult::kNotCanceled)
     return;
 
-  // TODO(yosin) We should check |targetPosition|, and
-  // |newSelection| are valid for |m_frame->document()|.
-  // |dispatchSelectStart()| can change them by "selectstart" event handler.
+  // |DispatchSelectStart()| can change |GetDocument()| or invalidate
+  // target_position by 'selectstart' event handler.
+  // TODO(editing-dev): We should also add a regression test when above
+  // behaviour happens. See crbug.com/775149.
+  if (!Selection().IsAvailable() || !target_position.IsValidFor(GetDocument()))
+    return;
 
   const bool should_extend_selection =
       selection_state_ == SelectionState::kExtendedSelection;