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This enforces a stricter patch read in a few ways:
- Raw Deltas of 0 are forbidden. These are unused values which have no
  meaning and should not appear.
- Extra Data length must be equal to the size of the patch element
  minus the total length of equivalences.
- Element match headers must have regions of nonzero length.

This change also annotates a number of "unsafe" values read from the
patch that could be sources of error later if unchecked. It also adds
caveats about under what conditions emitted values are considered to
be safe/unsafe.

Bug: 837096
Change-Id: I1b7614d92b85c0a1546d8dccb7d371d28b2a4cd3
Reviewed-on: https://chromium-review.googlesource.com/1037186
Commit-Queue: Calder Kitagawa <ckitagawa@google.com>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555396}