media/gpu/v4l2: Switch V4L2Buffer to use NativePixmapFrameResource

This changes the FrameResource backing type from VideoFrame to
NativePixmap when converting a V4L2-allocated buffer to a frame. This
changes the frame type for MTK devices, which encode into MMAP buffers
from the driver.

Bug: b:277581596
Test: Decode tests pass on Corsola.

Change-Id: I337162a5e78943fe23b78c4df1ddd72c569de103
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5229660
Reviewed-by: Fritz Koenig <frkoenig@chromium.org>
Commit-Queue: Nathan Hebert <nhebert@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1320094}