Update main WebFrame already exists DCHECK

This DCHECK occasionally triggers, but for unknown reasons. Update the
DCHECK to exclude re-registering the same main web frame. This will
separate the issue from the same frame being re-registered which will be
caught by the next, already existing DCHECK.

Bug: 1014227
Change-Id: Ic5a248f12467812e2b6b287565aa37f675935661
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354713
Auto-Submit: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798205}

ios/web/js_messaging/web_frames_manager_impl.mm

@@ -119,7 +119,8 @@ void WebFramesManagerImpl::AddFrame(std::unique_ptr<WebFrame> frame) {
   DCHECK(frame);
   DCHECK(!frame->GetFrameId().empty());
   if (frame->IsMainFrame()) {
-    DCHECK(!main_web_frame_);
+    DCHECK(!main_web_frame_ ||
+           (main_web_frame_->GetFrameId() == frame->GetFrameId()));
     main_web_frame_ = frame.get();
   }
   DCHECK(web_frames_.count(frame->GetFrameId()) == 0);