Clarify and simplify some code post review

Address post review comments from danakj after
https://chromium-review.googlesource.com/c/chromium/src/+/2044764
was submitted.

BUG=1046424

Change-Id: I8331fceb29ff12d2ec9eda28df9f8b147811de27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2062758Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742275}

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -2031,12 +2031,10 @@ WebLocalFrameImpl* WebViewImpl::MainFrameImpl() const {
 void WebViewImpl::DidAttachLocalMainFrame() {
   DCHECK(MainFrameImpl());
 
-  MainFrameImpl()->GetFrame()->WasAttachedAsLocalMainFrame();
+  LocalFrame* local_frame = MainFrameImpl()->GetFrame();
+  local_frame->WasAttachedAsLocalMainFrame();
 
-  MainFrameImpl()
-      ->GetFrame()
-      ->GetRemoteNavigationAssociatedInterfaces()
-      ->GetInterface(
+  local_frame->GetRemoteNavigationAssociatedInterfaces()->GetInterface(
       local_main_frame_host_remote_.BindNewEndpointAndPassReceiver());
 
   if (does_composite_) {

third_party/blink/renderer/core/exported/web_view_impl.h

@@ -705,7 +705,8 @@ class CORE_EXPORT WebViewImpl final : public WebView,
 
   Persistent<ResizeViewportAnchor> resize_viewport_anchor_;
 
-  // Handle to the local main frame host.
+  // Handle to the local main frame host. Only valid when the MainFrame is
+  // local.
   mojo::AssociatedRemote<mojom::blink::LocalMainFrameHost>
       local_main_frame_host_remote_;