Add MSE perf Telemetry tests.

tools/perf/page_sets/mse_cases.json

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