Merge to XFA: Make sure string constructors are efficient on literals

Besides the merge, there's one place where a wchar vs. wstring
comparison was being made that no longer compiled.

Original Review URL: https://codereview.chromium.org/1117263004

BUG=pdfium:151
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1116163003