PNaCl on-demand installs: Make a separate async IPC to check if PNaCl is installed

components/nacl/common/nacl_browser_delegate.h

Index: components/nacl/common/nacl_browser_delegate.h
diff --git a/components/nacl/common/nacl_browser_delegate.h b/components/nacl/common/nacl_browser_delegate.h
index 5655a8f8ba1c9a14622dfe12a522d3d979d40333..6cff071716e1c2ce12dc1ecdd64b9cfb2ede868a 100644
--- a/components/nacl/common/nacl_browser_delegate.h
+++ b/components/nacl/common/nacl_browser_delegate.h
@@ -58,6 +58,7 @@ class NaClBrowserDelegate {
   // Install PNaCl if this operation is supported. On success, the |installed|
   // callback should be called with true, and on failure (or not supported),
   // the |installed| callback should be called with false.
+  // TODO(jvoung): Add the progress callback as well.
   virtual void TryInstallPnacl(
       const base::Callback<void(bool)>& installed) = 0;
 };