[Blink] Remove use_gpu_rasterization check from VideoFrameImageUtil

The RasterContextProvider here comes from SharedGpuContext, as can be
seen by tracing back the value supplied at the two callsites [1][2].
SharedGpuContext creates the context provider here [3]. Creation can
either end up here [4][5] or here [6][7][8].

Hence, gpu rasterization is guaranteed to be enabled here.

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/webgpu/external_texture_helper.cc;l=432-435;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f
[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/video_frame_image_util.cc;l=224-228;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f?q=video_frame_image_util.cc&ss=chromium
[3] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/shared_gpu_context.cc;l=116-176;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f
[4] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/shared_gpu_context.cc;l=105-107;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f
[5] https://source.chromium.org/chromium/chromium/src/+/main:content/renderer/renderer_blink_platform_impl.cc;l=772-780;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f;bpv=1;bpt=1
[6] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/gpu/shared_gpu_context.cc;l=149-150;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f
[7] https://source.chromium.org/chromium/chromium/src/+/main:content/renderer/renderer_blink_platform_impl.cc;l=791;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f;bpv=1;bpt=1
[8] https://source.chromium.org/chromium/chromium/src/+/main:content/renderer/render_thread_impl.cc;l=1123-1133;drc=62a9a00176f862e688fa47919ed6f19b59f77b5f

Bug: 391648152
Change-Id: Iaa4211ca58e593dd53a9168fe9ab62c1ba7ef021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7031943
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1530010}