This reverts commit 19a8f1ba.

Reason for revert: it's actually slower this way

Original change's description:
> [scanner] Use AdvanceUntil in SkipWhiteSpace
> 
> Change-Id: I4a578589290e91da2eae4cef8f936e6c888e177c
> Reviewed-on: https://chromium-review.googlesource.com/1194015
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55497}

TBR=ishell@chromium.org,verwaest@chromium.org

Change-Id: I93b51104bef984332536360c6321eaa1dab8fd74
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1196402Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55513}