Purge the SkLiteDL freelist when PurgeAllCaches() is called.

Seems like the polite thing to do.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2228953002

Review-Url: https://codereview.chromium.org/2228953002
diff --git a/src/core/SkLiteDL.h b/src/core/SkLiteDL.h
index 072672d..a0c1685 100644
--- a/src/core/SkLiteDL.h
+++ b/src/core/SkLiteDL.h
@@ -21,6 +21,8 @@
 public:
     static sk_sp<SkLiteDL> New(SkRect);
 
+    static void PurgeFreelist();
+
     void optimizeFor(GrContext*);
     void makeThreadsafe();