Make SkSHA1 and SkM5 use common SkDigestHash result type

src/utils/SkHashDigest.h

Index: src/utils/SkHashDigest.h
===================================================================
--- src/utils/SkHashDigest.h	(revision 0)
+++ src/utils/SkHashDigest.h	(revision 0)
@@ -0,0 +1,74 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkHashDigest_DEFINED
+#define SkHashDigest_DEFINED
+
+#include "SkData.h"
+
+/**
+ * A mutable hash digest; it can be filled with a bytearray of any
+ * length, anytime.
+ */
+class SkHashDigest {
+public:
+    SkHashDigest() : fSkDataPtr(SkData::NewEmpty()) {}
+    SkHashDigest(const void *data, size_t length) : fSkDataPtr(SkData::NewWithCopy(data, length)) {}
+
+    /**
+     * Destructor: if this object is the only one with a reference to the data
+     * that makes up the hash digest, then that data will be freed.
+     */
+    ~SkHashDigest() {
+        SkSafeUnref(fSkDataPtr);
+    }
+
+    /**
+     * Replace the hash digest data held by this object with a copy of the
+     * data from this pointer/length.
+     */
+    void set(void *data, size_t length) {
+        fSkDataPtr->unref();
+        fSkDataPtr = SkData::NewWithCopy(data, length);
+    }
+
+    /**
+     * Return a pointer to the SkData object holding the hash digest data.
+     *
+     * The SkData object's reference counter will be bumped to account for this call,
+     * so the caller must call unref() on it once it is no longer needed!
+     *
+     * For example:
+     *   {
+     *     SkData *skDataPtr = digest.skDataPtr();
+     *     const uint8_t* bytes = skDataPtr->bytes();
+     *     ...
+     *     skDataPtr->unref();
+     *   }
+     */
+    SkData *skDataPtr() const {
+        fSkDataPtr->ref();
+        return fSkDataPtr;
+    }
+
+    size_t size() const {
+        return fSkDataPtr->size();
+    }
+
+    bool equals(const SkHashDigest &other) const {
+        return fSkDataPtr->equals(other.fSkDataPtr);
+    }
+
+private:
+
+    // fSkDataPtr may change over the lifetime of the SkHashDigest object,
+    // but it will never be set to NULL.
+    // (Every SkHashDigest constructor initializes it to point at *some* SkData object.)
+    SkData *fSkDataPtr;
+};
+
+#endif