headless: Allow both WW,HH and WWxHH forms for specifying window size

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 392c5f7e0d091c97e39c4900ac5c8ce233b9062f..06750596296047d2928b8986c97e7fcd47735b85 100644
--- a/headless/app/headless_shell_switches.cc
+++ b/headless/app/headless_shell_switches.cc
@@ -54,7 +54,7 @@ const char kUserDataDir[] = "user-data-dir";
 // specified virtual time budget is exhausted.
 const char kVirtualTimeBudget[] = "virtual-time-budget";
 
-// Sets the initial window size. Provided as string in the format "800x600".
+// Sets the initial window size. Provided as string in the format "800,600".
 const char kWindowSize[] = "window-size";
 
 }  // namespace switches