headless: expose window size through headless_shell switch.

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 7cc02ddb8db60f98bef0ea79e3ee219e2311895b..c748b9b133705dcb76c49fada18e738040cac9c6 100644
--- a/headless/app/headless_shell_switches.cc
+++ b/headless/app/headless_shell_switches.cc
@@ -34,5 +34,8 @@ const char kUseGL[] = "use-gl";
 // Directory where the browser stores the user profile.
 const char kUserDataDir[] = "user-data-dir";
 
+// Sets the initial window size. Provided as string in the format "800x600".
+const char kWindowSize[] = "window-size";
+
 }  // namespace switches
 }  // namespace headless