[Blink] Mark StaticBitmapImageTransform external draw as overwrite

The draw is scaled to the size of the CRP [1]. In addition, this
callsite creates the CRP locally to call DoExternalDrawAndSnapshot() and
then drops it. Thus, every draw is effectively an overwrite regardless,
since each CRP instance here is only drawn to once in any case.

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/graphics/static_bitmap_image_transform.cc;l=109-111;drc=15be50893f87a32a6a12adea5051b701414e932f

Bug: 352263194
Change-Id: I9a211ef1e1b63e6fca5edff317dd54bb7e8dd134
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7743309
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1612655}