fix picture subclass destructor doc

Failure to fully parse subclass destructor
caused bookmaker to drop everything that
followed. Fix it, and look for subclass
constructors more thoroughly.

TBR=caryclark@google.com
Docs-Preview: https://skia.org/?cl=152500
Bug: skia:6898
Change-Id: I623b9d5ea3f633e5e96276c2c9edc608f7b8c008
Reviewed-on: https://skia-review.googlesource.com/152500
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
6 files changed