Simplify UTF16LE_Encode and add unittest.

pdfium.gyp

 {
   'variables': {
     'pdf_use_skia%': 0,
   },
   'target_defaults': {
     'defines' : [
       'FT2_BUILD_LIBRARY',
       '_FPDFSDK_LIB',
       '_NO_GDIPLUS_',  # workaround text rendering issues on Windows
       'OPJ_STATIC',
@@ skipping 806 matching lines @@
         '<(DEPTH)/testing/gtest.gyp:gtest',
         'pdfium',
       ],
       'include_dirs': [
         '<(DEPTH)'
       ],
       'sources': [
         'testing/fx_string_testhelpers.h',
         'testing/fx_string_testhelpers.cpp',
         'core/src/fxcrt/fx_basic_bstring_unittest.cpp',
+        'core/src/fxcrt/fx_basic_wstring_unittest.cpp',
       ],
     },
     {
       'target_name': 'pdfium_embeddertests',
       'type': 'executable',
       'dependencies': [
         '<(DEPTH)/testing/gtest.gyp:gtest_main',
         '<(DEPTH)/testing/gtest.gyp:gtest',
         'pdfium',
       ],
       'include_dirs': [
         '<(DEPTH)'
       ],
       'sources': [
         'fpdfsdk/src/fpdfview_embeddertest.cpp',
         'testing/embedder_test.cpp',
         'testing/embedder_test.h',
         'testing/fx_string_testhelpers.cpp',
         'testing/fx_string_testhelpers.h',
       ],
     },
   ],
 }