[PDF Ink Signatures] Remove AnnotationBrushParams

The specification for Ink Signatures has changed slightly to allow the
eraser to have different sizes (https://crbug.com/349198718). Because
of this, all the expected brushes now must have a size.

Make the size field required in AnnotationBrush. Introduce a new
interface, Color, that is optional in AnnotationBrush, since the eraser
does not need color.

Bug: 349198718
Change-Id: Ic169200b295ee72c82817513e41325371d933f6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5685579
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1325581}