Remove unneeded check for is_stream_video

TextureDrawQuad::is_stream_video is set to true only in
VideoResourceUpdater [1]. That code always set background to
transparent.

SkPaint starts with alpha from DrawQuadParams::opacity [2]. If we don't
have RPDQ params, quad_alpha also starts as DrawParams::opacity [3].

The only place that can change it is when we blend with background
color, which doesn't happen because color is transparent. So calling
SkPaint::setAlpha is no op and we don't have to skip it.

[1]
https://source.chromium.org/chromium/chromium/src/+/main:media/renderers/video_resource_updater.cc;drc=8f5212f44f8566915586c12a7e9bc334ed5cde57;l=706
[2]
https://source.chromium.org/chromium/chromium/src/+/main:components/viz/service/display/skia_renderer.cc;drc=8636bb104c0e28b695adfe050dd1b70bfe05cdc8;l=567
[3]
https://source.chromium.org/chromium/chromium/src/+/main:components/viz/service/display/skia_renderer.cc;drc=8636bb104c0e28b695adfe050dd1b70bfe05cdc8;l=2721

Bug: 396584155
Change-Id: Iefb2b5f4c8d325c5fa84466e2080c1423db23f27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5839023
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Kyle Charbonneau <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1420715}