[Squad] Retrieve old ComputedStyle as const.

No need to use MutableComputedStyle there.

Bug: 813068
Change-Id: If1ce7d3048af3ae7085ad09dd04350a5d4f1dd67
Reviewed-on: https://chromium-review.googlesource.com/980537Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546094}

third_party/WebKit/Source/core/dom/Element.cpp

@@ -2198,7 +2198,7 @@ StyleRecalcChange Element::RecalcOwnStyle(StyleRecalcChange change) {
   DCHECK(change >= kIndependentInherit || NeedsStyleRecalc());
   DCHECK(ParentComputedStyle());
 
-  scoped_refptr<ComputedStyle> old_style = MutableComputedStyle();
+  scoped_refptr<const ComputedStyle> old_style = GetComputedStyle();
 
   // When propagating inherited changes, we don't need to do a full style recalc
   // if the only changed properties are independent. In this case, we can simply