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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}