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In https://crrev.com/612683 the blink-gen-property-trees flag was
changed from --enable-blink-gen-property-trees to
--enable-blink-features=BlinkGenPropertyTrees. This patch updates the
bot configuration to use the new flag.

Bug: 910859
Change-Id: I75bd3b947cf6fe5e8ec43a34a60e9cb7d61152c5
Reviewed-on: https://chromium-review.googlesource.com/c/1359131Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613250}