Finally got around to generating test media for this issue... and
it looks like the original fix was incomplete! We didn't read the
full box header before parsing CoLL and SmDm atoms.

R=cassew

Fixed: 1123430
Test: Updated unittest.
Change-Id: I7aca462f623cd12cb4842c2cfa1b0b9758ba2ab5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518898
Commit-Queue: Will Cassella <cassew@google.com>
Reviewed-by: Will Cassella <cassew@google.com>
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824595}