Remove mutex around image decoding in skpdiff.

tools/skpdiff/SkDiffContext.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkDiffContext_DEFINED
 #define SkDiffContext_DEFINED
 
@@ skipping 106 matching lines @@
     };
 
     struct DiffRecord {
         SkString           fCommonName;
         SkString           fDifferencePath;
         SkString           fBaselinePath;
         SkString               fTestPath;
         SkTArray<DiffData>        fDiffs;
     };
 
-    // This is needed to work around a bug in the multithreaded case where the
-    // image decoders are crashing when large numbers of threads are invoking
-    // the decoder at the same time.
-    // see https://code.google.com/p/skia/issues/detail?id=1803
-    SkMutex fImageMutex;
-
     // Used to protect access to fRecords and ensure only one thread is
     // adding new entries at a time.
     SkMutex fRecordMutex;
 
     // We use linked list for the records so that their pointers remain stable. A resizable array
     // might change its pointers, which would make it harder for async diffs to record their
     // results.
     SkTLList<DiffRecord> fRecords;
 
     SkImageDiffer** fDiffers;
     int fDifferCount;
     int fThreadCount;
 
     SkString fDifferenceDir;
 };
 
 #endif