Add a --virtual-time-budget option to headless_shell

headless/app/headless_shell_switches.cc

Index: headless/app/headless_shell_switches.cc
diff --git a/headless/app/headless_shell_switches.cc b/headless/app/headless_shell_switches.cc
index c748b9b133705dcb76c49fada18e738040cac9c6..a0afc6943108d3363719591f3b72e49d7d27e8ad 100644
--- a/headless/app/headless_shell_switches.cc
+++ b/headless/app/headless_shell_switches.cc
@@ -34,6 +34,14 @@ const char kUseGL[] = "use-gl";
 // Directory where the browser stores the user profile.
 const char kUserDataDir[] = "user-data-dir";
 
+// If set the system waits the specified number of virtual milliseconds before
+// deeming the page to be ready.  For determinism virtual time does not advance
+// while there are pending network fetches (i.e no timers will fire). Once all
+// network fetches have completed, timers fire and if the system runs out of
+// virtual time is fastforwarded so the next timer fires immediatley, until the
+// specified virtual time budget is exhausted.
+const char kVirtualTimeBudget[] = "virtual-time-budget";
+
 // Sets the initial window size. Provided as string in the format "800x600".
 const char kWindowSize[] = "window-size";