[mpc] Increase cost of style resolution in ExplicitInheritance.html

This test currently has a large number of cheap style resolves, which
is not appropriate for testing MatchedPropertiesCache effectiveness.

Hence this CL makes each style much more expensive by adding an
all:unset declaration. Also, the number of elements have been scaled
back such that the running time of the performance test stays
reasonable.

Bug: 1057072
Change-Id: Ib025d8d6b064d3924e4bc2d11285dc07d3949536
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2231350Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775719}

third_party/blink/perf_tests/css/ExplicitInheritance.html

@@ -6,10 +6,11 @@
 <style>
   .red { border: 1px solid red; }
   .green { border: 2px solid green; }
+  #container div { all: unset; }
   #container div { border-color: inherit; }
 </style>
 <script>
-  createDOMTree(container, 5, 6);
+  createDOMTree(container, 4, 5);
 
   PerfTestRunner.measureTime({
       description: 'Measure impact of explicit inheritance',