Move content's a plugin, ppapi_plugin, utility, and worker subdirectories to the content namespace.

content/plugin/plugin_channel.h

Index: content/plugin/plugin_channel.h
===================================================================
--- content/plugin/plugin_channel.h	(revision 163080)
+++ content/plugin/plugin_channel.h	(working copy)
@@ -17,6 +17,8 @@
 class WaitableEvent;
 }
 
+namespace content {
+
 // Encapsulates an IPC channel between the plugin process and one renderer
 // process.  On the renderer side there's a corresponding PluginChannelHost.
 class PluginChannel : public NPChannelBase {
@@ -94,4 +96,6 @@
   DISALLOW_COPY_AND_ASSIGN(PluginChannel);
 };
 
+}  // namespace content
+
 #endif  // CONTENT_PLUGIN_PLUGIN_CHANNEL_H_