SkPDF: SkBitSet gets reset() to make drop() better.

src/utils/SkBitSet.h

Index: src/utils/SkBitSet.h
diff --git a/src/utils/SkBitSet.h b/src/utils/SkBitSet.h
index a8585a20b9ea9d30820ca9ad7b3c64d1d5f52bdd..2fc9dda8108ce330684077bea411ce2aacea81d2 100644
--- a/src/utils/SkBitSet.h
+++ b/src/utils/SkBitSet.h
@@ -22,6 +22,11 @@ public:
     SkBitSet(const SkBitSet&) = delete;
     SkBitSet& operator=(const SkBitSet&) = delete;
 
+    void reset() {
+        fBitData = nullptr;
+        fDwordCount = 0;

[bungeman-skia, 2016/09/19 17:07:07]

This seems a bit strange. Previous to this fDwordCount and fBitData could have
been marked 'const', now they cannot.

+    }
+
     /** Set the value of the index-th bit to true.  */
     void set(int index) {
         uint32_t mask = 1 << (index & 31);