Enable a11y audit for chrome://print and fix failing tests.

chrome/test/data/webui/print_preview.h

Index: chrome/test/data/webui/print_preview.h
diff --git a/chrome/test/data/webui/print_preview.h b/chrome/test/data/webui/print_preview.h
index 26985638b346207e2281b7e567d1bbf9893ecc29..ea072f411d1f90e5eba78a26238f91c12a9c9e4b 100644
--- a/chrome/test/data/webui/print_preview.h
+++ b/chrome/test/data/webui/print_preview.h
@@ -16,4 +16,13 @@ class PrintPreviewWebUITest : public WebUIBrowserTest {
   DISALLOW_COPY_AND_ASSIGN(PrintPreviewWebUITest);
 };
 
+class PrintPreviewWebUIAsyncTest : public PrintPreviewWebUITest {
+ public:
+  PrintPreviewWebUIAsyncTest();
+  virtual ~PrintPreviewWebUIAsyncTest();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(PrintPreviewWebUIAsyncTest);
+};
+
 #endif  // CHROME_TEST_DATA_WEBUI_PRINT_PREVIEW_H_