Correct off-by-one in `Tokenizer::CanIncrement`.

It would return true even if you were actually at the end of the input.

Bug: 818844
Change-Id: I0ba5be52b568e39a087efc1e8114138ac4c18799
Reviewed-on: https://chromium-review.googlesource.com/949963
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#540982}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6783e46fa01a4da9c9b764d1858eb3c3f9b03f9e