Hypertext is empty on spin buttons

The hypertext interface is supported on kSpinButton, but
the hypertext is returned as "", and nCharacters=0.
This is causing issues with spin buttons in NVDA + Chrome.

The fix is to consider an input type=number as a text input field,
which allows the inner text to be computed from its contents.

Bug: 730965
Change-Id: Ib9c87b2481b86ac1a9f6d937fff7b0a7599109e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115245


Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915204}