Remove mutex around image decoding in skpdiff.

I removed the mutex to investigate BUG=skia:1803, and I get no
crashes. The crashes may have been related to other multithreading
issues.

Removing the mutex will allow skpdiff to run faster.

R=djsollen@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/61603008

git-svn-id: http://skia.googlecode.com/svn/trunk@12339 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed