PDF: Canon now owns a reference to all interned objects

Add SkPDFCanon::reset function to unref all objects.

No longer possible to remove object from canon

Motivation: this doesn't change these object's lifetime, (they will
still be fully unrefed when SkDocument::close() is called, but we no
longer have to remove them from the array when their destructor is
called.

Review URL: https://codereview.chromium.org/966863002