Re-enable property tree verification for layout tests
Re-enable property tree verification for layout tests

Property tree verification for layout tests had been disabled
while work was underway to make all layout tests pass with
compositor-thread property trees. Now that all layout tests
pass with verification enabled, this CL re-enables
verification.

BUG=509679

Review URL: https://codereview.chromium.org/1217823006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338745}

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