media/gpu/test: Cleanup on Intel Media compresison env var

This CL removes the comment about the browser process while setting up
the environment variable for Intel media compression as the test
environment VideoTestEnvironment at this point is used without it.

Also, this CL moves the setup of the environment variable after the
initialization of the feature list.

BUG=b:4599845, b:305794869, b:305964853
TEST=tast run $DUT video.ChromeStackDecoder.*, tast run $DUT
video.EncodeAccel

Change-Id: I435a569933f417e351b598c0c02ba18917353210
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4948812
Commit-Queue: Hang Nguyen <hnt@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1211088}