Clean up ChildProcessHost unique id generation.

content/public/common/child_process_host.h

Index: content/public/common/child_process_host.h
diff --git a/content/public/common/child_process_host.h b/content/public/common/child_process_host.h
index bccbd67b89d8d1683d0f80653b318e10e1ac4236..6ad0c291e37027a09b81ac65512d5eaf5e670773 100644
--- a/content/public/common/child_process_host.h
+++ b/content/public/common/child_process_host.h
@@ -24,9 +24,12 @@ class CONTENT_EXPORT ChildProcessHost : public IPC::Sender {
  public:
   virtual ~ChildProcessHost() {}
 
+  // This is a value never returned as the unique id of any child processes of
+  // any kind, including the values returned by RenderProcessHost::GetID().
+  static int kInvalidUniqueID;
+
   // Used to create a child process host. The delegate must outlive this object.
-  static ChildProcessHost* Create(
-      ChildProcessHostDelegate* delegate);
+  static ChildProcessHost* Create(ChildProcessHostDelegate* delegate);
 
   // These flags may be passed to GetChildPath in order to alter its behavior,
   // causing it to return a child path more suited to a specific task.