Add SkTHashSet::find()

BUG=skia:

Review URL: https://codereview.chromium.org/1058553002
diff --git a/tests/HashTest.cpp b/tests/HashTest.cpp
index 5abf3db..d2acafe 100644
--- a/tests/HashTest.cpp
+++ b/tests/HashTest.cpp
@@ -1,3 +1,10 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 #include "SkChecksum.h"
 #include "SkString.h"
 #include "SkTHash.h"
@@ -61,6 +68,9 @@
     REPORTER_ASSERT(r, set.contains(SkString("World")));
     REPORTER_ASSERT(r, !set.contains(SkString("Goodbye")));
 
+    REPORTER_ASSERT(r, set.find(SkString("Hello")));
+    REPORTER_ASSERT(r, *set.find(SkString("Hello")) == SkString("Hello"));
+
     set.reset();
     REPORTER_ASSERT(r, set.count() == 0);
 }