Bug 1216043 - Rename nsStyleSheet::sheetType and make it an enum class. r=dbaron

The only substantive change here, apart from a few variables changing in
size from uint16_t to uint8_t, is FontFaceSet's use of SheetType::Unknown
(0xFF) instead of 0 for FontFaceRecords for script-created FontFaces.

dom/base/nsDOMWindowUtils.cpp

@@ -3196,7 +3196,7 @@ convertSheetType(uint32_t aSheetType)
     default:
       NS_ASSERTION(false, "wrong type");
       // we must return something although this should never happen
-      return nsIDocument::SheetTypeCount;
+      return nsIDocument::AdditionalSheetTypeCount;
   }
 }