[PDF Ink Signatures] Reuse ConvertInkInputsIntoStroke() in Draw()

In InkModule, the existing ConvertInkInputsIntoStroke() and Draw()
methods have nearly identical code. Refactor
ConvertInkInputsIntoStroke() to remove its side effects, so it can
become a const CreateInProgressStrokeFromInputs() method, and let the
callers use the result to perform the side effects as needed.

This makes the code a little bit cleaner, and makes it a bit easier to
implement the eraser tool.

Bug: 335524381
Change-Id: I49333791aca0a003d4f47feb0820efc747b30b6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5564066
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Alan Screen <awscreen@chromium.org>
Reviewed-by: Andy Phan <andyphan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1305396}