Change WebCursor::GetPlatformCursor color type DCHECK to NOTIMPLEMENTED

CreateCursorFromSkBitmap returns empty cursors for incompatible types.
Replace the DCHECK with a NOTIMPLEMENTED in CreateCursorFromSkBitmap.
(I can't repro with 24/8/4/1-bpp bmps, see the bug...)
If a savvy user/dev hits this, we can investigate further.

Bug: 955398
Test: No custom cursor behavior differences, logging instead of dcheck.
Change-Id: Ie1f774ded98e6d9fa2b20731a6995a08c89f8bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583127
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654194}
2 files changed