media/gpu: V4L2SVD & VaapiVD: Update visible_rect after decoding.

Originally we use visible_rect and natural_size from
VideoDecoderConfig passed from VD::Initialize(). But the resolution
might be changed during decoding. This CL uses the value of visible
size passed from AVD and calculate natural_size from the ratio.

BUG=chromium:941330
TEST=Run video_decode_tests with switch_1080p_720p_240frames.h264
     on Kevin and Eve
TEST=./media_unittests --gtest_filter=PlatformVideoFramePoolTest*

Change-Id: I63e285d14733732b5ec8f77691224a75badea50a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1657857
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Auto-Submit: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: David Staessens <dstaessens@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676416}