vaapi: Verify H.264 encoder UpdateRates() with SW rate controller

This CL adds test to H264VaapiVideoEncoderDelegateTest suite to verify
that VA-API H.264 encoder delegate update rates correctly with the
software bitrate controller.

Bug: 195407733, 348661735
Test: H264VaapiVideoEncoderDelegateTest in media_unittests
Change-Id: I3bad829c2043ca5b938f621be655213e9dfa9b0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5827722
Commit-Queue: Hang Nguyen <hnt@chromium.org>
Reviewed-by: Justin Green <greenjustin@google.com>
Cr-Commit-Position: refs/heads/main@{#1350847}