[css-grid] Prettify the nested-grid.html perf test

Grid items have now a fixed height so that the grid could be seen when
opening the test with a browser.

This artificially improves the runs/s of the test because a fixed height is
much cheaper to compute than an auto height, but we do not plan to change it
so the test will still be sensitive to layout regressions.

Review-Url: https://codereview.chromium.org/2345963002
Cr-Commit-Position: refs/heads/master@{#419154}

third_party/WebKit/PerformanceTests/Layout/nested-grid.html

@@ -14,6 +14,7 @@ html, body {
     grid-auto-columns: 1fr;
 }
 
+.gridItem { height: 10px; }
 </style>
 
 <script src="../resources/runner.js"></script>