Remove null checks and early outs for closing in RenderWidget.

RenderWidget used to start closing and then post a task to self-delete.
But now it deletes synchronously inside Close(). So when closing_
becomes true, the RenderWidget will be deleted in the same stack. Thus
we do not need to guard against closing_ since blink will not be using
the RenderWidget afterward - it would be a UAF.

The LayerTreeViewDelegate methods used to check for a null WebWidget
which would be the case once closing_ became true, before RenderWidget
was destroyed. Now the RenderWidget disconnects itself from the
LayerTreeView and deletes immediately, so these methods are never
called with a null WebWidget unless they were used while the
RenderWidget is undead. But the compositor does not run while the
RenderWidget is undead, and the LayerTreeViewDelegate will not be used
unless the compositor posted the task and then runs it after the
RenderWidget becomes undead. The methods in this CL are all part of the
BeginMainFrame step which only runs when the compositor is visible and
the RenderWidget is not undead.

R=avi@chromium.org

Bug: 419087
Change-Id: If0158f2ffeaf0c5d334a80aed3cdb9e686002fb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854878Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705178}

content/renderer/render_widget.h

@@ -654,10 +654,6 @@ class CONTENT_EXPORT RenderWidget
                        int relative_cursor_pos);
   void OnImeFinishComposingText(bool keep_selection);
 
-  // This does the actual focus change, but is called in more situations than
-  // just as an IPC message.
-  void SetFocus(bool enable);
-
   // Called by the browser process to update text input state.
   void OnRequestTextInputStateUpdate();
 
@@ -879,13 +875,6 @@ class CONTENT_EXPORT RenderWidget
   // Used to force the size of a window when running web tests.
   void SetWindowRectSynchronously(const gfx::Rect& new_window_rect);
 
-  // A variant of Send but is fatal if it fails. The browser may
-  // be waiting for this IPC Message and if the send fails the browser will
-  // be left in a state waiting for something that never comes. And if it
-  // never comes then it may later determine this is a hung renderer; so
-  // instead fail right away.
-  void SendOrCrash(IPC::Message* msg);
-
   // Determines whether or not RenderWidget should process IME events from the
   // browser. It always returns true unless there is no WebFrameWidget to
   // handle the event, or there is no page focus.