Move most of the remaining users of WebContentsObserver::tab_contents() to use web_contents().

chrome/browser/ui/sad_tab_observer.cc

Index: chrome/browser/ui/sad_tab_observer.cc
===================================================================
--- chrome/browser/ui/sad_tab_observer.cc	(revision 115777)
+++ chrome/browser/ui/sad_tab_observer.cc	(working copy)
@@ -39,7 +39,7 @@
     return;
 
   gfx::NativeView view = AcquireSadTab(status);
-  tab_contents()->GetView()->InstallOverlayView(view);
+  web_contents()->GetView()->InstallOverlayView(view);
 }
 
 void SadTabObserver::Observe(int type,
@@ -48,7 +48,7 @@
   switch (type) {
     case content::NOTIFICATION_TAB_CONTENTS_CONNECTED:
       if (HasSadTab()) {
-        tab_contents()->GetView()->RemoveOverlayView();
+        web_contents()->GetView()->RemoveOverlayView();
         ReleaseSadTab();
       }
       break;
@@ -73,12 +73,12 @@
   // and later re-parent it.
   // TODO(avi): This is a cheat. Can this be made cleaner?
   sad_tab_params.parent_widget =
-      static_cast<TabContentsViewViews*>(tab_contents()->GetView());
+      static_cast<TabContentsViewViews*>(web_contents()->GetView());
   sad_tab_params.ownership =
       views::Widget::InitParams::WIDGET_OWNS_NATIVE_WIDGET;
   sad_tab_.reset(new views::Widget);
   sad_tab_->Init(sad_tab_params);
-  sad_tab_->SetContentsView(new SadTabView(tab_contents(), kind));
+  sad_tab_->SetContentsView(new SadTabView(web_contents(), kind));
   return sad_tab_->GetNativeView();
 #elif defined(TOOLKIT_GTK)
   sad_tab_.reset(new SadTabGtk(