Remove unnecessary null checks in LocalDOMWindow::InstallNewDocument.

Change-Id: I9d4c9bf676be75bc2b6bec5d18515de092b99d6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616946
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841718}

third_party/blink/renderer/core/frame/local_dom_window.cc

@@ -609,25 +609,29 @@ bool LocalDOMWindow::HasInsecureContextInAncestors() {
 }
 
 Document* LocalDOMWindow::InstallNewDocument(const DocumentInit& init) {
-  DCHECK_EQ(init.GetWindow(), this);
+  // Blink should never attempt to install a new Document to a LocalDOMWindow
+  // that's not attached to a LocalFrame.
+  DCHECK(GetFrame());
+  // Either:
+  // - `this` should be a new LocalDOMWindow, that has never had a Document
+  //   associated with it or
+  // - `this` is being reused, and the previous Document has been disassociated
+  //   via `ClearForReuse()`.
   DCHECK(!document_);
+  DCHECK_EQ(init.GetWindow(), this);
+
   document_ = init.CreateDocument();
   document_->Initialize();
 
-  if (!GetFrame())
-    return document_;
-
   GetScriptController().UpdateDocument();
   document_->GetViewportData().UpdateViewportDescription();
-  if (FrameScheduler* frame_scheduler = GetFrame()->GetFrameScheduler()) {
+
+  auto* frame_scheduler = GetFrame()->GetFrameScheduler();
   frame_scheduler->TraceUrlChange(document_->Url().GetString());
   frame_scheduler->SetCrossOriginToMainFrame(
       GetFrame()->IsCrossOriginToMainFrame());
-  }
 
-  if (GetFrame()->GetPage() && GetFrame()->View()) {
   GetFrame()->GetPage()->GetChromeClient().InstallSupplements(*GetFrame());
-  }
 
   return document_;
 }