Fix bad return value in WideString::Compare().

When comparing L"a" vs L"ab", the return value should be -1 or 1, not 0
or 1. Add a regression test for this case.

BUG=chromium:821454

Change-Id: I38e2d7ca62319b7a62f8d8afeb231b8ed3bd9e86
Reviewed-on: https://pdfium-review.googlesource.com/28570
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
diff --git a/core/fxcrt/widestring.cpp b/core/fxcrt/widestring.cpp
index 8d47564..7b5bf66 100644
--- a/core/fxcrt/widestring.cpp
+++ b/core/fxcrt/widestring.cpp
@@ -924,7 +924,7 @@
     return result;
   if (this_len == that_len)
     return 0;
-  return this_len < that_len;
+  return this_len < that_len ? -1 : 1;
 }
 
 int WideString::CompareNoCase(const wchar_t* lpsz) const {