PDF without annotations skips structure tree.

The SkPDFDocument::fTagTree::fRoot is set at document creation time.
This root will initially always be discardable, since no annotations
have been made. As a result it does not make sense to assert on it being
non-discardable, since it should be possible to create a PDF which just
happens to have no annotations added to it.

Change-Id: I2fe336c872805b6937f7c8ea275c0cb4d3438682
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/301383
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
2 files changed