[PDF Ink Signatures] Refactor MaybeSetDrawingBrushAndCursor()

Remove the MaybeSetCursor() call in MaybeSetDrawingBrushAndCursor() and
rename it to only MaybeSetDrawingBrush(). Have the callers handle
calling MaybeSetCursor() instead. This prevents the method from doing
too much and enables https://crrev.com/c/6459883 to set the drawing
brush without setting the cursor.

There should be no visible change in behavior.

Bug: 410761158
Change-Id: I0ea51e2f1c565ad71905317d4ac2b3fe2c049cd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6498549
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1453466}