[chromedriver] Return not-implemented status code for FullscreenWindow command.

BUG=chromedriver:1617

Review-Url: https://codereview.chromium.org/2533963002
Cr-Commit-Position: refs/heads/master@{#435068}

chrome/test/chromedriver/server/http_handler.cc

@@ -341,6 +341,11 @@ HttpHandler::HttpHandler(
           kPost,
           "session/:sessionId/window/:windowHandle/maximize",
           WrapToCommand("MaximizeWindow", base::Bind(&ExecuteMaximizeWindow))),
+      CommandMapping(
+          kPost,
+          "session/:sessionId/window/fullscreen",
+          WrapToCommand("FullscreenWindow",
+                        base::Bind(&ExecuteUnimplementedCommand))),
       CommandMapping(kDelete,
                      "session/:sessionId/window",
                      WrapToCommand("CloseWindow", base::Bind(&ExecuteClose))),