make picture-imagegenerator more robust on requested infos

This new unittest would assert before this fix.

Bug: skia:6501
Change-Id: I351ad03f29bccc054f72bfcb838174830dbd008c
Reviewed-on: https://skia-review.googlesource.com/13413
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
2 files changed