Simplify UTF16LE_Encode and add unittest.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 
 pdf_use_skia = false
 
 config("pdfium_config") {
   cflags = []
@@ skipping 779 matching lines @@
     "fpdfsdk/src/formfiller/FFL_TextField.cpp",
     "fpdfsdk/src/formfiller/FFL_Utils.cpp",
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
 }
 
 test("pdfium_unittests") {
   sources = [
     "core/src/fxcrt/fx_basic_bstring_unittest.cpp",
+    "core/src/fxcrt/fx_basic_wstring_unittest.cpp",
     "testing/fx_string_testhelpers.cpp",
     "testing/fx_string_testhelpers.h",
   ]
   deps = [
     "//testing/gtest",
     "//testing/gtest:gtest_main",
     ":pdfium"
   ]
   include_dirs = [ "." ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ ":pdfium_config", "//build/config/compiler:no_chromium_code" ]
 }