telemetry: Added timeout parameter to StartTracing.

tools/telemetry/telemetry/core/chrome/tracing_backend.py

Index: tools/telemetry/telemetry/core/chrome/tracing_backend.py
diff --git a/tools/telemetry/telemetry/core/chrome/tracing_backend.py b/tools/telemetry/telemetry/core/chrome/tracing_backend.py
index 633afb2a76b34ac6e07bae06a72cd0960d05c33c..644544c8b75499c037b6ba4e7cb75459d5d0df99 100644
--- a/tools/telemetry/telemetry/core/chrome/tracing_backend.py
+++ b/tools/telemetry/telemetry/core/chrome/tracing_backend.py
@@ -54,12 +54,12 @@ class TracingBackend(object):
     self._thread = None
     self._tracing_data = []
 
-  def BeginTracing(self, custom_categories=None):
+  def BeginTracing(self, custom_categories=None, timeout=10):
     self._CheckNotificationSupported()
     req = {'method': 'Tracing.start'}
     if custom_categories:
       req['params'] = {'categories': custom_categories}
-    self._SyncRequest(req)
+    self._SyncRequest(req, timeout)
     # Tracing.start will send asynchronous notifications containing trace
     # data, until Tracing.end is called.
     self._thread = threading.Thread(target=self._TracingReader)