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

This Cl updates the usage documentation for the video_encode_accelerator_tests
and video_encode_accelerator_perf_tests with the new "--codec" parameter.

TEST=gitiles

BUG=None

Change-Id: I1bfed4af765f2b4ac8ce4323f2209c29e710a382
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215656
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773568}

docs/media/gpu/video_encoder_test_usage.md

@@ -53,10 +53,11 @@ 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:
 
+    --codec              codec profile to encode, "h264 (baseline)",
+                         "h264main, "h264high", "vp8" and "vp9"
      -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.