harden requirements on SkBitmapCache

src/lazy/SkCachingPixelRef.cpp

Index: src/lazy/SkCachingPixelRef.cpp
diff --git a/src/lazy/SkCachingPixelRef.cpp b/src/lazy/SkCachingPixelRef.cpp
index 6d97aaeee97e6ca1d2984ab5de0bcef04b7f7e5c..56431880c9f2dc2225f14176e035439d80e8f104 100644
--- a/src/lazy/SkCachingPixelRef.cpp
+++ b/src/lazy/SkCachingPixelRef.cpp
@@ -54,6 +54,7 @@ bool SkCachingPixelRef::onNewLockPixels(LockRec* rec) {
             fErrorInDecoding = true;
             return false;
         }
+        fLockedBitmap.setImmutable();
         SkBitmapCache::Add(this->getGenerationID(), info.fWidth, info.fHeight, fLockedBitmap);
     }