PlzNavigate: use is_waiting_for_beforeunload_ack in the RenderFrameHost

This CL removes the function IsWaitingForBeforeUnloadACK in favor of just using
the is_waiting_for_beforeunload_ack now that PlzNavigate uses the
BeforeUnload IPC. This CL undoes lots of
the work done in https://codereview.chromium.org/789643005.

BUG=504347

Review URL: https://codereview.chromium.org/1213773003

Cr-Commit-Position: refs/heads/master@{#336543}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 2d64c53..5ee8b9c 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3993,7 +3993,7 @@
   if (DevToolsAgentHost::IsDebuggerAttached(this))
     return;
 
-  if (rfhi->IsWaitingForBeforeUnloadACK() ||
+  if (rfhi->is_waiting_for_beforeunload_ack() ||
       rfhi->IsWaitingForUnloadACK()) {
     // Hang occurred while firing the beforeunload/unload handler.
     // Pretend the handler fired so tab closing continues as if it had.
@@ -4009,7 +4009,7 @@
     // close. Otherwise, pretend the unload listeners have all fired and close
     // the tab.
     bool close = true;
-    if (rfhi->IsWaitingForBeforeUnloadACK() && delegate_) {
+    if (rfhi->is_waiting_for_beforeunload_ack() && delegate_) {
       delegate_->BeforeUnloadFired(this, true, &close);
     }
     if (close)