Move RenderViewHostObserver to content/public/browser/ and put it into the content namespace.

chrome/browser/extensions/extension_message_handler.h

Index: chrome/browser/extensions/extension_message_handler.h
diff --git a/chrome/browser/extensions/extension_message_handler.h b/chrome/browser/extensions/extension_message_handler.h
index 5fa9a18a3f5ad637b5e599cf5b6d1110e9423cf6..9ace4aed27937a5346bd008e9a4a45e697547de7 100644
--- a/chrome/browser/extensions/extension_message_handler.h
+++ b/chrome/browser/extensions/extension_message_handler.h
@@ -8,7 +8,7 @@
 
 #include <string>
 
-#include "content/browser/renderer_host/render_view_host_observer.h"
+#include "content/public/browser/render_view_host_observer.h"
 
 // Filters and dispatches extension-related IPC messages that arrive from
 // renderers. There is one of these objects for each RenderViewHost in Chrome.
@@ -19,7 +19,7 @@
 // could eliminate this class. Right now, we don't end up with an EFD for tab
 // contents unless that tab contents is hosting chrome-extension:// URLs. That
 // still leaves content scripts. See also: crbug.com/80307.
-class ExtensionMessageHandler : public RenderViewHostObserver {
+class ExtensionMessageHandler : public content::RenderViewHostObserver {
  public:
   // |sender| is guaranteed to outlive this object.
   explicit ExtensionMessageHandler(RenderViewHost* render_view_host);