Bug 1558442 - Use a weaker change hint for auto -> non-auto offset changes. r=dbaron

See the review comments in bug 1558442 to see why these should be correct.
Quoting:

> I think it would be good to avoid passing ClearDescendantIntrinsics and
> NeedDirtyReflow here, since those are the most expensive ones, and I think
> they're pretty clearly not needed. I think ClearAncestorIntrinsics is also
> clearly not needed, since I'm reasonably confident that absolutely positioned
> kids don't contribute to their parent's intrinsic size.

I think the same thing applies to "slow" offset changes (when autoness changes),
since relative movement doesn't affect intrinsic sizes either.

Differential Revision: https://phabricator.services.mozilla.com/D34497

--HG--
extra : moz-landing-system : lando

layout/style/nsStyleStruct.cpp

@@ -1512,7 +1512,7 @@ nsChangeHint nsStylePosition::CalcDifference(
       hint |=
           nsChangeHint_RecomputePosition | nsChangeHint_UpdateParentOverflow;
     } else {
-      hint |= nsChangeHint_AllReflowHints;
+      hint |= nsChangeHint_NeedReflow | nsChangeHint_ReflowChangesSizeOrPosition;
     }
   }
   return hint;