just add SkLiteDL::empty()

TBR=djsollen@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2249713002

Review-Url: https://codereview.chromium.org/2249713002
diff --git a/src/core/SkLiteDL.cpp b/src/core/SkLiteDL.cpp
index 73d6d16..d649889 100644
--- a/src/core/SkLiteDL.cpp
+++ b/src/core/SkLiteDL.cpp
@@ -755,6 +755,10 @@
     return fBounds;
 }
 
+bool SkLiteDL::empty() const {
+    return fUsed == 0;
+}
+
 #if !defined(SK_LITEDL_USES)
     #define  SK_LITEDL_USES 16
 #endif
diff --git a/src/core/SkLiteDL.h b/src/core/SkLiteDL.h
index 0798366..27e3479 100644
--- a/src/core/SkLiteDL.h
+++ b/src/core/SkLiteDL.h
@@ -25,6 +25,7 @@
 
     void optimizeFor(GrContext*);
     void makeThreadsafe();
+    bool empty() const;
 
     void save();
     void saveLayer(const SkRect*, const SkPaint*, const SkImageFilter*, SkCanvas::SaveLayerFlags);