Use explicit WebString conversions in LayoutTest related files

components/test_runner/mock_grammar_check.cc

Index: components/test_runner/mock_grammar_check.cc
diff --git a/components/test_runner/mock_grammar_check.cc b/components/test_runner/mock_grammar_check.cc
index 912576624d7e2e1e88ab7007f6ad650b9c3ea208..b1bdcebc2675af018e10514f9e3957430dd41002 100644
--- a/components/test_runner/mock_grammar_check.cc
+++ b/components/test_runner/mock_grammar_check.cc
@@ -20,7 +20,7 @@ bool MockGrammarCheck::CheckGrammarOfString(
     const blink::WebString& text,
     std::vector<blink::WebTextCheckingResult>* results) {
   DCHECK(results);
-  base::string16 string_text = text;
+  base::string16 string_text = text.utf16();
   if (std::find_if(string_text.begin(), string_text.end(), IsASCIIAlpha) ==
       string_text.end())
     return true;