vea_unittest: add level as input property and test it

Add level as a input test stream property to specify requested level. Currently
it is only valid for H264 codec. ForceLevel is implemented to check whether the
output level satisfies the requested one (only for H264 now). Moreover, in
order to test lower level (ex. Level1) a new test stream with the video size
128x96 is added to media/test/data.

In addition to verifying the level confiuration, it is also required to specify
the higher level (ex. Level5.1) for vea_unittest when we want to test 4k
(2160p) video encoding for 4k-supported platforms.

BUG=886564
TEST=build and run video_encode_accelerator_unittest on Eve and Elm
TEST=./video_encode_accelerator_unittest --test_stream_data=bear_320x192_40frames.yuv:320:192:1:out1.h264:200000:30::::40
TEST=./video_encode_accelerator_unittest --test_stream_data=bear_128x96_40frames.yuv:128:96:1:out1.h264:64000:30::::10
TEST=use 3840x2160 raw video as vea_unittest input with Level5.1 on Eve (2160p video is too big to upload to media/test/data)

Change-Id: I68512f5e848a35abbe0c3af802d0926261780241
Reviewed-on: https://chromium-review.googlesource.com/c/1469809Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Pin-chih Lin <johnylin@chromium.org>
Auto-Submit: Pin-chih Lin <johnylin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635066}

media/test/data/README.md

@@ -750,6 +750,11 @@ The frame sizes change between 1080p and 720p every 24 frames.
 
 ### VEA test files:
 
+#### bear_128x96_40frames.yuv
+First 40 raw i420 frames of bear-1280x720.mp4 scaled down to 128x96 for
+video_encode_accelerator_unittest. This is the size that could be encoded
+with the lowest H264 level 1.0 in 30 fps.
+
 #### bear_320x192_40frames.yuv
 First 40 raw i420 frames of bear-1280x720.mp4 scaled down to 320x192 for
 video_encode_accelerator_unittest.