media/gpu/test: Add usage documentation for new video encoder tests.

This CL adds usage documentation for the new video_encode_accelerator_tests,
providing in-depth information on how to run the encoder tests.

TEST=gitiles

BUG=953114

Change-Id: Iaacaa2e6c79417e69eedced9c21b63ed9f483f16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060437
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771725}

docs/media/gpu/video_encoder_test_usage.md

+# Video Encoder tests
+The video encoder tests are a set of tests that validate various video encoding
+scenarios. They run directly on top of the video encoder implementation, and
+don't require the full Chrome browser stack. They are built on top of the
+[GoogleTest](https://github.com/google/googletest/blob/master/README.md)
+framework.
+
+__Note:__ Currently these tests are under development and functionality is still
+limited.
+
+[TOC]
+
+## Running from Tast
+Running these tests from Tast is not supported yet.
+
+## Running manually
+To run the video encoder tests manually the _video_encode_accelerator_tests_
+target needs to be built and deployed to the device being tested. Running
+the video encoder tests can be done by executing:
+
+    ./video_encode_accelerator_tests [<video path>] [<video metadata path>]
+
+e.g.: `./video_encode_accelerator_tests bear_320x192_40frames.yuv.webm`
+
+__Test videos:__ Various test videos are present in the
+[_media/test/data_](https://cs.chromium.org/chromium/src/media/test/data/)
+folder in Chromium's source tree (e.g.
+[_bear_320x192_40frames.yuv.webm_](https://cs.chromium.org/chromium/src/media/test/data/bear_320x192_40frames.yuv.webm)).
+These videos are stored in compressed format and extracted at the start of each
+test run, as storing uncompressed videos requires a lot of disk space. Currently
+only VP9 or uncompressed videos are supported as test input. If no video is
+specified _bear_320x192_40frames.yuv.webm_ will be used.
+
+__Video Metadata:__ These videos also have an accompanying metadata _.json_ file
+that needs to be deployed alongside the test video. The video metadata file is a
+simple json file that contains info about the video such as its pixel format,
+dimensions, framerate and number of frames. These can also be found in the
+_media/test/data_ folder (e.g.
+[_bear_320x192_40frames.yuv.webm.json_](https://cs.chromium.org/chromium/src/media/test/data/bear_320x192_40frames.yuv.webm.json)).
+If no metadata file is specified _\<video path\>.json_ will be used.
+
+## Command line options
+Multiple command line arguments can be given to the command:
+
+     -v                  enable verbose mode, e.g. -v=2.
+    --vmodule            enable verbose mode for the specified module,
+                         e.g. --vmodule=*media/gpu*=2.
+
+    --gtest_help         display the gtest help and exit.
+    --help               display this help and exit.
+
+## Source code
+See the video encoder tests [source code](https://cs.chromium.org/chromium/src/media/gpu/video_encode_accelerator_tests.cc).
+