Longhand/Shorthand should use CSSProperty::Flags instead of uint16_t

We are approaching our current 16-bit limit for flags, and last time
I exceeded this limit locally (and hence redefined CSSProperty::Flags
to uint32_t), it took a while to understand why things didn't work.
The reason was that the Longhand/Shorthand constructors truncated the
integer to uint16_t.

No new flags are planned at the moment, but I still want to eliminate
this trap for when we _do_ add a new flag.

Change-Id: I4527b7ed16b9f289135a3a37665cbcf319d8bb7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124252Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754432}

third_party/blink/renderer/core/css/properties/longhand.h

@@ -43,9 +43,7 @@ class Longhand : public CSSProperty {
   }
 
  protected:
-  constexpr Longhand(CSSPropertyID id,
-                     uint16_t flags,
-                     char repetition_separator)
+  constexpr Longhand(CSSPropertyID id, Flags flags, char repetition_separator)
       : CSSProperty(id, flags | kLonghand, repetition_separator) {}
 };
 

third_party/blink/renderer/core/css/properties/shorthand.h

@@ -31,9 +31,7 @@ class Shorthand : public CSSProperty {
   }
 
  protected:
-  constexpr Shorthand(CSSPropertyID id,
-                      uint16_t flags,
-                      char repetition_separator)
+  constexpr Shorthand(CSSPropertyID id, Flags flags, char repetition_separator)
       : CSSProperty(id, flags | kShorthand, repetition_separator) {}
 };