Remove duplication between TestRunner's fields and LayoutDumpFlags struct.

components/test_runner/web_test_runner.h

Index: components/test_runner/web_test_runner.h
diff --git a/components/test_runner/web_test_runner.h b/components/test_runner/web_test_runner.h
index b5040dff2f7cf37b2163c5aed262650fd9e22e19..612baf75fca2fbd1124768c23bb31769c50afe10 100644
--- a/components/test_runner/web_test_runner.h
+++ b/components/test_runner/web_test_runner.h
@@ -8,14 +8,14 @@
 #include <string>
 #include <vector>
 
-#include "components/test_runner/layout_dump_flags.h"
-
 namespace blink {
 class WebContentSettingsClient;
 }
 
 namespace test_runner {
 
+struct LayoutDumpFlags;
+
 class WebTestRunner {
  public:
   // Returns a mock WebContentSettings that is used for layout tests. An
@@ -33,7 +33,7 @@ class WebTestRunner {
 
   // Gets layout dump flags (i.e. dump-as-text or dump-as-markup) requested
   // by the test (i.e. via testRunner.dumpAsText() called from javascript).
-  virtual LayoutDumpFlags GetLayoutDumpFlags() = 0;
+  virtual const LayoutDumpFlags& GetLayoutDumpFlags() = 0;
 
   // If custom text dump is present (i.e. if testRunner.setCustomTextOutput has
   // been called from javascript), then returns |true| and populates the