[ios blink] Fix build failures with gfx::NativeWindow

On some platforms like iOS gfx::NativeWindow is not a raw ptr
so initialization with nullptr is incorrect. Ensure the
gfx::NativeWindow type is properly initialized.

Fallout from https://chromium-review.googlesource.com/c/chromium/src/+/6283547

Bug: 40254930
Change-Id: Ice33cc8951e7d7e968edf06762401b6b1dea9fb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6287816
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1423208}