SkImage method for detecting lazy decoding

BUG=skia:4224
R=reed@google.com

Review URL: https://codereview.chromium.org/1305453007
diff --git a/include/core/SkPixelRef.h b/include/core/SkPixelRef.h
index 5d1aef7..0d9182a 100644
--- a/include/core/SkPixelRef.h
+++ b/include/core/SkPixelRef.h
@@ -265,6 +265,11 @@
 
     virtual SkDiscardableMemory* diagnostic_only_getDiscardable() const { return NULL; }
 
+    /**
+     *  Returns true if the pixels are generated on-the-fly (when required).
+     */
+    bool isLazyGenerated() const { return this->onIsLazyGenerated(); }
+
 protected:
     /**
      *  On success, returns true and fills out the LockRec for the pixels. On
@@ -318,6 +323,8 @@
 
     virtual bool onRequestLock(const LockRequest&, LockResult*);
 
+    virtual bool onIsLazyGenerated() const { return false; }
+
     /** Return the mutex associated with this pixelref. This value is assigned
         in the constructor, and cannot change during the lifetime of the object.
     */