SkLiteDL: inline empty()

1.85%     1.85%  RenderThread     6458  6475  /system/lib64/libskia.so  SkLiteDL::empty() const

:/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2254823002

Review-Url: https://codereview.chromium.org/2254823002
diff --git a/src/core/SkLiteDL.h b/src/core/SkLiteDL.h
index d53c8de..27b6e92 100644
--- a/src/core/SkLiteDL.h
+++ b/src/core/SkLiteDL.h
@@ -24,7 +24,7 @@
 
     void optimizeFor(GrContext*);
     void makeThreadsafe();
-    bool empty() const;
+    bool empty() const { return fUsed == 0; }
 
     void save();
     void saveLayer(const SkRect*, const SkPaint*, const SkImageFilter*, SkCanvas::SaveLayerFlags);