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

ppapi/api/private/ppb_nacl_private.idl

Index: ppapi/api/private/ppb_nacl_private.idl
diff --git a/ppapi/api/private/ppb_nacl_private.idl b/ppapi/api/private/ppb_nacl_private.idl
index 9e4e898abe941137e65e8f917803d77728843ae1..1054723c85cb6badc7c54766ee130e0f4de1ac77 100644
--- a/ppapi/api/private/ppb_nacl_private.idl
+++ b/ppapi/api/private/ppb_nacl_private.idl
@@ -79,10 +79,16 @@ interface PPB_NaCl_Private {
                                 [in] uint32_t desired_access,
                                 [in] uint32_t options);
 
+  /* Check if PNaCl is installed and attempt to install if necessary.
+   * Callback is called when the check is done and PNaCl is already installed,
+   * or after an on-demand install is attempted. Called back with PP_OK if
+   * PNaCl is available. Called back with an error otherwise.
+   */
+  int32_t EnsurePnaclInstalled([in] PP_Instance instance,
+                               [in] PP_CompletionCallback callback);
+
   /* Returns a read-only file descriptor of a file rooted in the Pnacl
    * component directory, or an invalid handle on failure.
-   * Do we want this to take a completion callback and be async, or
-   * could we make this happen on another thread?
    */
   PP_FileHandle GetReadonlyPnaclFd([in] str_t filename);