Avoid copying value in ComputedStyle CoW comparions (compareEqual)

Because of the cast of the RHS, |u|, a copy would be generated, with
code and cycle bloat as the result. In some cases this can even have
prevented inlining. Particularly nasty examples:

 bool compareEqual(const Vector<LengthPoint>&, const Vector<LengthPoint>&) [210 bytes]
 bool compareEqual(const Vector<CSSPropertyID>&, const Vector<CSSPropertyID>&) [274 bytes]
 bool compareEqual(const Vector<GridTrackSize>&, const Vector<GridTrackSize>&) [182 bytes]
 bool compareEqual(const HashMap<String, GridArea>&, const HashMap<String, GridArea>&) [308 bytes]
 bool compareEqual(const TransformOperations&, const TransformOperations&) [441 bytes]
 bool compareEqual(const LengthBox&, const LengthBox&) [249 bytes]

Remove the U->T cast in compareEqual to avoid the copies. Nothing seems
to require this coercion (anymore?) This eliminates the above symbols
entirely. A total binary size reduction of >30k (x86-64; non-official) also
indicates even the simpler cases/types may have benefited.

Review-Url: https://chromiumcodereview.appspot.com/2438353002
Cr-Commit-Position: refs/heads/master@{#426990}

third_party/WebKit/Source/core/style/ComputedStyle.h

@@ -82,6 +82,11 @@ inline bool compareEqual(const T& t, const U& u) {
   return t == static_cast<T>(u);
 }
 
+template <typename T>
+inline bool compareEqual(const T& a, const T& b) {
+  return a == b;
+}
+
 #define SET_VAR(group, variable, value)      \
   if (!compareEqual(group->variable, value)) \
   group.access()->variable = value