Add MSE perf Telemetry tests.

tools/perf/page_sets/mse_cases.json

+{
+  "description": "Media source extensions perf benchmark",
+  "test-action": [
+    { "action": "wait", "javascript" : "window.__testDone() == true"}
+  ],
+  "pages": [
+    {
+      "url": "file://mse_cases/startup_test.html?testType=AV",
+      "media_metrics": {"action": "test-action"}

[dtu, 2013/11/22 23:30:20]

You can just define media_metrics at the top, where test-action is. Pages
inherit all top-level properties.

[shadi, 2013/11/23 00:01:06]

Good point. Done.

+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=AV&useAppendStream=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=AV&doNotWaitForBodyOnLoad=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=AV&useAppendStream=true&doNotWaitForBodyOnLoad=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=V",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=V&useAppendStream=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=V&doNotWaitForBodyOnLoad=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=V&useAppendStream=true&doNotWaitForBodyOnLoad=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=A",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=A&useAppendStream=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=A&doNotWaitForBodyOnLoad=true",
+      "media_metrics": {"action": "test-action"}
+    },
+    {
+      "url": "file://mse_cases/startup_test.html?testType=A&useAppendStream=true&doNotWaitForBodyOnLoad=true",
+      "media_metrics": {"action": "test-action"}
+    }
+  ]
+}