This places the hot part of SkipWhiteSpace in the header, allowing it to be
inlined, and leaves a slow path to handle the rest. This improves comment
scanning overall by ~10%.

Bug: v8:7926
Change-Id: I2e2ebbbae0d1af619b161397712fdf667d078884
Reviewed-on: https://chromium-review.googlesource.com/1150230
Commit-Queue: Florian Sattler <sattlerf@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54744}