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This adds video frame flags, --frame_validator=,
check : compares md5 values computed from video frame with the expected md5
        values. Test fails if there is an unexpected video frame.
dump  : outputs video frames to files

BUG=chromium:856562
TEST=VDA unittest with --test_import
TEST=VDA unittest with --test_import --frame_validator=check
TEST=VDA unittest with --frame_validator=check
TEST=VDA unittest with --frame_validator=check,dump
TEST=VDA unittest with --frame_validator=dump
TEST=VDA unittest with --gtest_filter=VideoDecodeAcceleratorTest.DISABLED_GenMD5 --gtest_also_run_disabled_tests

Change-Id: I1511ef07a2bd01231e9daf1038776324c22e3da5
Reviewed-on: https://chromium-review.googlesource.com/c/1309390Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605598}