Bug 1430817: Serialize <overflow-position> before other align bits. r=mats

Otherwise the serialisation wouldn't roundtrip with the new syntax, which fixes
the position of <overflow-position>.

Also make Servo and Gecko agree on whether to serialize "unsafe".

MozReview-Commit-ID: L3GSMk5pZ3F

layout/style/nsCSSValue.cpp

@@ -1265,7 +1265,11 @@ nsCSSValue::AppendAlignJustifyValueToString(int32_t aValue, nsAString& aResult)
     aValue &= ~legacy;
     aResult.AppendLiteral("legacy ");
   }
+  // Don't serialize the 'unsafe' keyword; it's the default.
   auto overflowPos = aValue & (NS_STYLE_ALIGN_SAFE | NS_STYLE_ALIGN_UNSAFE);
+  if (MOZ_UNLIKELY(overflowPos == NS_STYLE_ALIGN_SAFE)) {
+    aResult.AppendLiteral("safe ");
+  }
   aValue &= ~overflowPos;
   MOZ_ASSERT(!(aValue & NS_STYLE_ALIGN_FLAG_BITS),
              "unknown bits in align/justify value");
@@ -1283,12 +1287,6 @@ nsCSSValue::AppendAlignJustifyValueToString(int32_t aValue, nsAString& aResult)
   }
   const auto& kwtable(nsCSSProps::kAlignAllKeywords);
   AppendASCIItoUTF16(nsCSSProps::ValueToKeyword(aValue, kwtable), aResult);
-  // Don't serialize the 'unsafe' keyword; it's the default.
-  if (MOZ_UNLIKELY(overflowPos == NS_STYLE_ALIGN_SAFE)) {
-    aResult.Append(' ');
-    AppendASCIItoUTF16(nsCSSProps::ValueToKeyword(overflowPos, kwtable),
-                       aResult);
-  }
 }
 
 /**