Don't read beyond a StringPiece's bounds in StringToNumber()

This change was included in
https://chromium-review.googlesource.com/c/chromium/src/+/947745
and was reverted by
https://chromium-review.googlesource.com/c/chromium/src/+/950343
but is still needed by
https://chromium-review.googlesource.com/c/chromium/src/+/945049

Bug: chromium:817982, chromium:818376
Change-Id: I4f97d632ba9f86c17d3df55c404c0ae14e1d39c9
Reviewed-on: https://chromium-review.googlesource.com/950466
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Joshua Peraza <jperaza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541183}
7 files changed