Add property flags for which properties apply to certain pseudo-eleme…

…nts. r=emilio

These flags can be used to flag properties that apply to ::first-letter, ::first-line, and ::placeholder

Part 1 of the fix for Gecko bug 1382786 <https://bugzilla.mozilla.org/show_bug.cgi?id=1382786>.

components/style/properties/properties.mako.rs

@@ -442,6 +442,12 @@ bitflags! {
         const ABSPOS_CB = 1 << 2,
         /// This shorthand property is an alias of another property.
         const SHORTHAND_ALIAS_PROPERTY = 1 << 3,
+        /// This longhand property applies to ::first-letter.
+        const APPLIES_TO_FIRST_LETTER = 1 << 4,
+        /// This longhand property applies to ::first-line.
+        const APPLIES_TO_FIRST_LINE = 1 << 5,
+        /// This longhand property applies to ::placeholder.
+        const APPLIES_TO_PLACEHOLDER = 1 << 6,
     }
 }