Speculative fix for crash in RWHVCF::SetTooltipText().

Since the use of GetCursorManager() came after this bug was filed, let's
assume we're hitting either a null frame_connector_ or the root view
is unavailable.

Bug: 831972
Change-Id: I9868fca6872c15a7b178fbb6b5d489dabfdf4729
Reviewed-on: https://chromium-review.googlesource.com/1023166Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552834}

content/browser/renderer_host/render_widget_host_view_child_frame.cc

@@ -437,9 +437,14 @@ void RenderWidgetHostViewChildFrame::Destroy() {
 
 void RenderWidgetHostViewChildFrame::SetTooltipText(
     const base::string16& tooltip_text) {
-  frame_connector_->GetRootRenderWidgetHostView()
-      ->GetCursorManager()
-      ->SetTooltipTextForView(this, tooltip_text);
+  if (!frame_connector_)
+    return;
+
+  auto* root_view = frame_connector_->GetRootRenderWidgetHostView();
+  if (!root_view)
+    return;
+
+  root_view->GetCursorManager()->SetTooltipTextForView(this, tooltip_text);
 }
 
 RenderWidgetHostViewBase* RenderWidgetHostViewChildFrame::GetParentView() {