[Fuchsia] Fix CHECK() when shutting down web.Frame.

FrameImpl was destroying FocusController without removing it from list
of window event handlers, which was causing CHECK in the destructor.

Bug: 906785
Change-Id: I78bb13a60e9e56ce663cfba9f6ded0357c3bab4c
Reviewed-on: https://chromium-review.googlesource.com/c/1344749Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609799}

webrunner/browser/frame_impl.cc

@@ -172,6 +172,7 @@ FrameImpl::~FrameImpl() {
   if (window_tree_host_) {
     aura::client::SetFocusClient(root_window(), nullptr);
     wm::SetActivationClient(root_window(), nullptr);
+    root_window()->RemovePreTargetHandler(focus_controller_.get());
     web_contents_->ClosePage();
     window_tree_host_->Hide();
     window_tree_host_->compositor()->SetVisible(false);