Handle zero-length encoded images gracefully during deserialization

Image encoding may fail during serialization, resulting in zero-length
encoded data in the SKP.

Instead of invalidating the stream (and preventing deserialization of
the whole picture) we can instantiate placeholder images.

BUG=skia:4285
R=reed@google.com,robertphillips@google.com

Review URL: https://codereview.chromium.org/1308273011
4 files changed