Enforce Peek() before Consume().

In the streaming parser, you can never Consume() without knowing
that you will not consume a block start token. Enforce this
with a DCHECK (which, incidentally, found two such bugs).
It will not take everything (e.g., there is still at least one
calc() parsing bug left), but at least requires more complicated
patterns to create such bugs.

Change-Id: I60191c874b8981479cc983e17fc958e2a6eeb66b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5861248
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356975}