Implement FPDF_VIEWERREF_GetName() API.

This is a generic API function to retrieve any viewer preference of type
name.

BUG=pdfium:414

Review-Url: https://codereview.chromium.org/2475923003
diff --git a/public/fpdf_doc.h b/public/fpdf_doc.h
index 0b78743..c3be0e0 100644
--- a/public/fpdf_doc.h
+++ b/public/fpdf_doc.h
@@ -271,7 +271,7 @@
 // Returns the number of bytes in the title, including trailing zeros.
 //
 // The |buffer| is always encoded in UTF-16LE. The |buffer| is followed by two
-// bytes of zeros indicating the end of the string.  If |buflen| is less then
+// bytes of zeros indicating the end of the string.  If |buflen| is less than
 // the returned length, or |buffer| is NULL, |buffer| will not be modified.
 DLLEXPORT unsigned long STDCALL FPDF_GetMetaText(FPDF_DOCUMENT doc,
                                                  FPDF_BYTESTRING tag,