Move string16 and char16 in to base::

minidump/test/minidump_string_writer_test_util.h

Index: minidump/test/minidump_string_writer_test_util.h
diff --git a/minidump/test/minidump_string_writer_test_util.h b/minidump/test/minidump_string_writer_test_util.h
index 2749214266c6d5a26aa621247b610b4570118e09..dcac2a3593b3673e85bf21d66bb2099f8a11ce20 100644
--- a/minidump/test/minidump_string_writer_test_util.h
+++ b/minidump/test/minidump_string_writer_test_util.h
@@ -78,7 +78,8 @@ const MinidumpUTF8String* MinidumpUTF8StringAtRVA(
 //!     failure, raises a gtest assertion and returns an empty string.
 //!
 //! \sa MinidumpUTF8StringAtRVAAsString()
-string16 MinidumpStringAtRVAAsString(const std::string& file_contents, RVA rva);
+base::string16 MinidumpStringAtRVAAsString(const std::string& file_contents,
+                                           RVA rva);
 
 //! \brief Returns the contents of a MinidumpUTF8String as a `std::string`.
 //!