Make video encoder/decoder test faster

Reduced size of the frames used in the tests to make them faster.
Previously all Video tests in remoting_unittests were taking more than
10 seconds, now thay take less than 4

BUG=514404

Review URL: https://codereview.chromium.org/1264743002

Cr-Commit-Position: refs/heads/master@{#341206}
2 files changed