Check that mainframes don't fail to be swapped out.

When this fails the proxy is deleted but the RenderFrame of the
speculative frame would be left in a shutting down state would not
delete the RenderFrame in the renderer.

R=dcheng@chromium.org

Bug: 939262
Change-Id: I38947b3471a2ba510d59f6132d286376473ba787
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529165Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642111}

content/renderer/render_frame_impl.cc

@@ -2330,10 +2330,14 @@ void RenderFrameImpl::OnSwapOut(
   // This executes the unload handlers on this frame and its local descendants.
   bool success = frame_->Swap(proxy->web_frame());
 
-  // For main frames, the swap should have cleared the RenderView's pointer to
-  // this frame.
-  if (is_main_frame)
+  if (is_main_frame) {
+    // TODO(crbug.com/939262): Looking for ways a main frame could be left in a
+    // provisional state and not deleted by the browser.
+    CHECK(success);
+    // For main frames, the swap should have cleared the RenderView's pointer to
+    // this frame.
     CHECK(!render_view->main_render_frame_);
+  }
 
   if (!success) {
     // The swap can fail when the frame is detached during swap (this can