Fix the code that causes warnings

core/fxcrt/fx_basic_wstring_unittest.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/fxcrt/include/fx_basic.h"
 #include "testing/fx_string_testhelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 TEST(fxcrt, WideStringOperatorSubscript) {
   // CFX_WideString includes the NUL terminator for non-empty strings.
@@ skipping 673 matching lines @@
   for (size_t i = 0; i < FX_ArraySize(utf16le_encode_cases); ++i) {
     EXPECT_EQ(utf16le_encode_cases[i].bs,
               utf16le_encode_cases[i].ws.UTF16LE_Encode())
         << " for case number " << i;
   }
 }
 
 TEST(fxcrt, WideStringCOperatorSubscript) {
   // CFX_WideStringC includes the NUL terminator for non-empty strings.
   CFX_WideStringC abc(L"abc");
-  EXPECT_EQ(L'a', abc[0]);
-  EXPECT_EQ(L'b', abc[1]);
-  EXPECT_EQ(L'c', abc[2]);
-  EXPECT_EQ(L'\0', abc[3]);
+  EXPECT_EQ(L'a', abc.CharAt(0));
+  EXPECT_EQ(L'b', abc.CharAt(1));
+  EXPECT_EQ(L'c', abc.CharAt(2));
+  EXPECT_EQ(L'\0', abc.CharAt(3));
 }
 
 TEST(fxcrt, WideStringCOperatorLT) {
   CFX_WideStringC empty;
   CFX_WideStringC a(L"a");
   CFX_WideStringC abc(L"\x0110qq");  // Comes before despite endianness.
   CFX_WideStringC def(L"\x1001qq");  // Comes after despite endianness.
 
   EXPECT_FALSE(empty < empty);
   EXPECT_FALSE(a < a);
@@ skipping 224 matching lines @@
   }
 }
 
 TEST(fxcrt, EmptyWideString) {
   CFX_WideString empty_str;
   EXPECT_TRUE(empty_str.IsEmpty());
   EXPECT_EQ(0, empty_str.GetLength());
   const FX_WCHAR* cstr = empty_str.c_str();
   EXPECT_EQ(0, FXSYS_wcslen(cstr));
 }