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The test appears to get stuck on waiting for black frames. This patch
adds some logging so we perhaps can learn more where in the image we
find the nonblack pixels. Also trying to increase tolerance for
rounding errors somewhat to see if that helps.

BUG=477498

Review URL: https://codereview.chromium.org/1108803002

Cr-Commit-Position: refs/heads/master@{#329616}