Allow GMs ato be used as benchmarks. Make convex_poly_clip opt in.

bench/SkGMBench.h

Index: bench/SkGMBench.h
diff --git a/bench/SkGMBench.h b/bench/SkGMBench.h
new file mode 100644
index 0000000000000000000000000000000000000000..925a87583c12683e93b41f190aa0d2d375a6aeb3
--- /dev/null
+++ b/bench/SkGMBench.h
@@ -0,0 +1,31 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkBenchmark.h"
+#include "gm.h"
+#include "SkCanvas.h"
+
+/**
+ * Runs a GM as a benchmark by repeatedly drawing the GM.
+ */
+class SkGMBench : public SkBenchmark {
+public:
+    // Constructor takes ownership of the GM param.
+    SkGMBench(skiagm::GM* gm);
+    virtual ~SkGMBench();
+
+protected:
+    virtual const char* onGetName() SK_OVERRIDE;
+    virtual bool isSuitableFor(Backend backend) SK_OVERRIDE;
+    virtual void onDraw(const int loops, SkCanvas* canvas) SK_OVERRIDE;
+    virtual SkIPoint onGetSize() SK_OVERRIDE;
+
+private:
+    skiagm::GM* fGM;
+    SkString    fName;
+    typedef SkBenchmark INHERITED;
+};