Implement a more stable way to check if the dialog is fully loaded before choose the sink, instead …

chrome/test/media_router/telemetry/benchmarks/pagesets/media_router_page.py

Index: chrome/test/media_router/telemetry/benchmarks/pagesets/media_router_page.py
diff --git a/chrome/test/media_router/telemetry/benchmarks/pagesets/media_router_page.py b/chrome/test/media_router/telemetry/benchmarks/pagesets/media_router_page.py
index 5c5223d1cfb2b9152e3f396b6fdd48f3e3f27f18..25d11f3acef9aafa92979214f77d65696a1e4d78 100644
--- a/chrome/test/media_router/telemetry/benchmarks/pagesets/media_router_page.py
+++ b/chrome/test/media_router/telemetry/benchmarks/pagesets/media_router_page.py
@@ -91,6 +91,21 @@ class CastPage(page.Page):
     self._WaitForResult(action_runner, verify_func, error_message,
                         timeout=timeout)
 
+  def WaitUntilDialogLoaded(self, action_runner, tab):
+    """Waits until dialog is fully loaded."""
+
+    self._WaitForResult(
+        action_runner,
+        lambda: tab.EvaluateJavaScript(
+             '!!window.document.getElementById('
+             '"media-router-container") &&'
+             'window.document.getElementById('
+             '"media-router-container").sinksToShow_ &&'
+             'window.document.getElementById('
+             '"media-router-container").sinksToShow_.length'),
+        'The dialog is not fully loaded within 15s.',
+         timeout=15)
+
   def _WaitForResult(self, action_runner, verify_func, error_message,
                      timeout=5):
     """Waits until the function finishes or timeout."""