Simplify UTF16LE_Encode and add unittest.

core/src/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 "testing/gtest/include/gtest/gtest.h"
+#include "../../../testing/fx_string_testhelpers.h"
+#include "../../include/fxcrt/fx_basic.h"
+
+TEST(fxcrt, WideStringUTF16LE_Encode) {
+  CFX_WideString wide_strings[] = {
+          L"",
+          L"abc",
+          L"abcdef",
+          L"abc\0def",
+          L"23\0456",

[Tom Sepez, 2015/01/21 18:15:54]

beware of tailing digits following a \0, the intent is clearer when uing alphas,
which is covered above. this case doesn't add much beyond that, so we can pull
it.

Can we also get a case where the high bit is set in each byte of the result to
check for sign-extension weirdness?

[Bo Xu, 2015/01/21 19:23:55]

Done.

+          L"\x3132\x6162"  // This is wrong? Endianness matters here?

[Tom Sepez, 2015/01/21 18:15:54]

So, what you'd have here is a wide string:
  U+3132, U+6162
which is being formatted as ut16 little endian, which I'd
expect to see as:
0x32, 0x31, 0x62, 0x61 or 21ba.

[Bo Xu, 2015/01/21 19:23:55]

Oh, right, i see.

+  };
+  CFX_ByteString byte_strings[] = {

[Tom Sepez, 2015/01/21 18:15:54]

nit: its preferable to make a single array of structures out of these so that
they don't get out of sync with each other.  Also, feel free to use a macro to
get rid of boiler plate initializations.

[Bo Xu, 2015/01/21 19:23:55]

Done.

+          CFX_ByteString(FX_BSTRC("\0\0")),
+          CFX_ByteString(FX_BSTRC("a\0b\0c\0\0\0")),
+          CFX_ByteString(FX_BSTRC("a\0b\0c\0d\0e\0f\0\0\0")),
+          CFX_ByteString(FX_BSTRC("a\0b\0c\0\0\0")),
+          CFX_ByteString(FX_BSTRC("\x32\x00\x33\x00\045\x00\x36\x00\x00\x00")),
+          CFX_ByteString(FX_BSTRC("12ab\0\0"))
+  };
+  for (size_t i = 0; i < FX_ArraySize(wide_strings); ++i) {
+          EXPECT_EQ(byte_strings[i], wide_strings[i].UTF16LE_Encode())
+                  << " for case number " << i;
+  }
+}