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Reland: vea_unittest: Calculate per-frame encode latency

This is a reland of https://crrev.com/1117853002, which failed to compile
in CHECK_GT(size_t var, 0), and CHECK_LE(size_t var, 100) because the compiler
handles the numeric literals as signed. So compiler complained about comparing
signed and unsigned. This is fixed by using unsigned numeric literals like 0UL
and 100UL.

Calculates per-frame encode latency, which is basically the time delay from
input of each VideoFrame (VEA::Encode()) to output of the corresponding
BitstreamBuffer (VEA::Client::BitstreamBufferReady()).

It calculates encode latency values at the 50th(median), 75th, and 95th percentiles.

Also fix a wrong comment.

BUG=345181
TEST=Check "Encode latency for the 50%/75%/95%" in output logs.

Review URL: https://codereview.chromium.org/1139913009

Cr-Commit-Position: refs/heads/master@{#330060}