Remove mojo::String.

mojo/public/cpp/bindings/tests/wtf_types_unittest.cc

Index: mojo/public/cpp/bindings/tests/wtf_types_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/wtf_types_unittest.cc b/mojo/public/cpp/bindings/tests/wtf_types_unittest.cc
index d18241e56848829aee7291de44ba0e8bf39273e7..be8283f5f40efd1f4988aa4947d5089fa627235b 100644
--- a/mojo/public/cpp/bindings/tests/wtf_types_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/wtf_types_unittest.cc
@@ -167,11 +167,11 @@ TEST_F(WTFTypesTest, Serialization_WTFArrayToMojoArray) {
   mojo::internal::Serialize<MojomType>(strs, &buf, &data, &validate_params,
                                        &context);
 
-  Array<mojo::String> strs2;
+  Array<std::string> strs2;

[yzshen1, 2017/01/03 21:44:41]

Array<base::Optional<std::string>>?

[Sam McNally, 2017/01/03 23:44:28]

Done.

   mojo::internal::Deserialize<MojomType>(data, &strs2, &context);
 
   ASSERT_EQ(4u, strs2.size());
-  EXPECT_TRUE(strs2[0].is_null());
+  EXPECT_TRUE(strs2[0].empty());
   EXPECT_TRUE("" == strs2[1]);
   EXPECT_TRUE(kHelloWorld == strs2[2]);
   EXPECT_TRUE(kUTF8HelloWorld == strs2[3]);