harden requirements on SkBitmapCache

BUG=skia:
R=humper@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/521433002
diff --git a/src/lazy/SkCachingPixelRef.cpp b/src/lazy/SkCachingPixelRef.cpp
index 6d97aae..5643188 100644
--- a/src/lazy/SkCachingPixelRef.cpp
+++ b/src/lazy/SkCachingPixelRef.cpp
@@ -54,6 +54,7 @@
             fErrorInDecoding = true;
             return false;
         }
+        fLockedBitmap.setImmutable();
         SkBitmapCache::Add(this->getGenerationID(), info.fWidth, info.fHeight, fLockedBitmap);
     }