image_extractor: return None if expected image is not found.

BUG=chromium:348889
TEST=unittests

Change-Id: I2bf4927de35dbe09a5e4a8b79838dc308517608e
Reviewed-on: https://chromium-review.googlesource.com/188757
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
2 files changed