when we disable lcd for impl reasons, add kGenA8 to document the original intent

BUG=skia:
R=bungeman@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/572933004
diff --git a/include/core/SkDevice.h b/include/core/SkDevice.h
index 618431a..f55354b 100644
--- a/include/core/SkDevice.h
+++ b/include/core/SkDevice.h
@@ -138,8 +138,7 @@
     };
 
     struct TextFlags {
-        uint32_t            fFlags;     // SkPaint::getFlags()
-        SkPaint::Hinting    fHinting;
+        uint32_t    fFlags;     // SkPaint::getFlags()
     };
 
     /**