Move a bunch of code in content\common (as well as a few left in renderer) to the content namespace.

content/renderer/plugin_channel_host.h

Index: content/renderer/plugin_channel_host.h
===================================================================
--- content/renderer/plugin_channel_host.h	(revision 163290)
+++ content/renderer/plugin_channel_host.h	(working copy)
@@ -9,9 +9,11 @@
 #include "content/common/np_channel_base.h"
 #include "ipc/ipc_channel_handle.h"
 
-class IsListeningFilter;
 class NPObjectBase;
 
+namespace content {
+class IsListeningFilter;
+
 // Encapsulates an IPC channel between the renderer and one plugin process.
 // On the plugin side there's a corresponding PluginChannel.
 class PluginChannelHost : public NPChannelBase {
@@ -78,4 +80,6 @@
   DISALLOW_COPY_AND_ASSIGN(PluginChannelHost);
 };
 
+}  // namespace content
+
 #endif  // CONTENT_RENDERER_PLUGIN_CHANNEL_HOST_H_