Instead of using explicit current_, next_, and next_next_ pointers we now use a
range from token_start_ to token_end_. Upon scan we "precrement" token_end_ to
update the scan_target(). This will allow us to increase the buffer size later.

Change-Id: I816d208eafc33c29a9888e7040696b56b1409fb8
Reviewed-on: https://chromium-review.googlesource.com/1189984Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55450}