Add gpu_perftests to "Android Nexus5 Perf".

BUG=464373

Review URL: https://codereview.chromium.org/981063003

Cr-Commit-Position: refs/heads/master@{#319839}

testing/buildbot/chromium.perf.json

@@ -12,6 +12,11 @@
       {
         "name": "host_info",
         "script": "host_info.py"
+      },
+      {
+	"name": "gpu_perftests",
+	"script": "gtest_perf_test.py",
+	"args": ["gpu_perftests"]
       }
     ]
   },