Make Editor::CanCopy() to work with detached IFRAME

This patch makes |Editor::CanCopy()| to work with detached IFRAME to follow up
of the patch[1], which found this issue.

[1] http://crrev.com/575324: Use a safer pattern for auto resetting
RenderFrameImpl members

Bug: 746181
Change-Id: If00b1d22476264a042806ff9870ca32229041908
Reviewed-on: https://chromium-review.googlesource.com/578913
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488580}

content/test/data/detach_frame_in_copy.html

@@ -5,9 +5,6 @@
 <script>
 window[0].addEventListener('copy', (e) => {
   document.querySelector('iframe').remove();
-  // TODO(dcheng): We really shouldn't do this but it turns out there are bugs
-  // in editing code. See https://crbug.com/746181 ^_^
-  e.preventDefault();
   // Finish asynchronously, to make sure the renderer doesn't crash.
   window.setTimeout(() => { document.title = 'done'; });
 });

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

@@ -354,6 +354,8 @@ bool Editor::CanCopy() const {
   if (ImageElementFromImageDocument(GetFrame().GetDocument()))
     return true;
   FrameSelection& selection = GetFrame().Selection();
+  if (!selection.IsAvailable())
+    return false;
   return selection.ComputeVisibleSelectionInDOMTreeDeprecated().IsRange() &&
          !IsInPasswordFieldWithUnrevealedPassword(
              GetFrame().Selection().ComputeVisibleSelectionInDOMTree().Start());