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Since Math.random() in python generates a random number in range [0,1),
we need to use rand(256) to generate number [0, 255]. Update all calls
in canvas perftest to call rand(256).

Bug:1087019

Change-Id: I75ded3269cf652d7d352e32eefbfeddb999583b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216736
Commit-Queue: Yi Xu <yiyix@chromium.org>
Reviewed-by: Juanmi Huertas <juanmihd@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772294}