Clean up URL test string conversions.

Originally the url test utils had UTF string conversion functions because we
didn't want the library to depend on STL (for embedders). We've not had this
limitation for a long time so the UTF16 <--> UTF8 conversion functions can be
deleted in favor of the ones in base.

The one remaining function which generates possibly-invalid UTF16 (by design)
is renamed and clarified for this purpose.

Review-Url: https://codereview.chromium.org/2469133002
Cr-Commit-Position: refs/heads/master@{#429688}
diff --git a/url/url_util_unittest.cc b/url/url_util_unittest.cc
index af5d5500..a706f5a08 100644
--- a/url/url_util_unittest.cc
+++ b/url/url_util_unittest.cc
@@ -214,15 +214,15 @@
     RawCanonOutputT<base::char16> output;
     DecodeURLEscapeSequences(input, strlen(input), &output);
     EXPECT_EQ(decode_cases[i].output,
-              test_utils::ConvertUTF16ToUTF8(base::string16(output.data(),
-                                                            output.length())));
+              base::UTF16ToUTF8(base::string16(output.data(),
+                                               output.length())));
   }
 
   // Our decode should decode %00
   const char zero_input[] = "%00";
   RawCanonOutputT<base::char16> zero_output;
   DecodeURLEscapeSequences(zero_input, strlen(zero_input), &zero_output);
-  EXPECT_NE("%00", test_utils::ConvertUTF16ToUTF8(
+  EXPECT_NE("%00", base::UTF16ToUTF8(
       base::string16(zero_output.data(), zero_output.length())));
 
   // Test the error behavior for invalid UTF-8.