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

gyp/bench.gyp

Index: gyp/bench.gyp
diff --git a/gyp/bench.gyp b/gyp/bench.gyp
index d404000fcece0d5d0308588cdc6d30fda708b15a..c408d7cb406312f632a9206d12571b96ae59cebe 100644
--- a/gyp/bench.gyp
+++ b/gyp/bench.gyp
@@ -97,6 +97,9 @@
         '../bench/SkBenchLogger.h',
         '../bench/SkBenchmark.cpp',
         '../bench/SkBenchmark.h',
+        '../bench/SkGMBench.cpp',
+        '../bench/SkGMBench.h',
+
         '../bench/benchmain.cpp',
       ],
       'conditions': [
@@ -111,6 +114,9 @@
           },
         ],
       ],
+      'includes': [
+        'gmslides.gypi',
+      ],
     },
     {
       'target_name' : 'bench_timer',