headless: Require the user to pass in an initial URL

headless/public/headless_browser.h

Index: headless/public/headless_browser.h
diff --git a/headless/public/headless_browser.h b/headless/public/headless_browser.h
index 75394078f87d3fb90d170d7acf69a5c9fb60fa0d..367a8384fec3957b274be81238ed40eaeebc4927 100644
--- a/headless/public/headless_browser.h
+++ b/headless/public/headless_browser.h
@@ -35,8 +35,10 @@ class HEADLESS_EXPORT HeadlessBrowser {
  public:
   struct Options;
 
-  // Create a new browser tab. |size| is in physical pixels.
+  // Create a new browser tab which navigates to |initial_url|. |size| is in
+  // physical pixels.
   virtual std::unique_ptr<HeadlessWebContents> CreateWebContents(
+      const GURL& initial_url,
       const gfx::Size& size) = 0;
 
   // Returns a task runner for submitting work to the browser main thread.