[Telemetry] Set user agent through Network.setUserAgentOverride instead of --user-agent.

tools/telemetry/telemetry/user_agent.py

Index: tools/telemetry/telemetry/user_agent.py
diff --git a/tools/telemetry/telemetry/user_agent.py b/tools/telemetry/telemetry/user_agent.py
index 6a19e58c7e98816dcf57f6cd72ca29fbe67186f5..93bc118658dbfb670fda6337e65daa7d434665a2 100644
--- a/tools/telemetry/telemetry/user_agent.py
+++ b/tools/telemetry/telemetry/user_agent.py
@@ -18,11 +18,11 @@ UA_TYPE_MAPPING = {
 }
 
 
-def GetChromeUserAgentArgumentFromType(user_agent_type):
-  """Returns a chrome user agent based on a user agent type.
+def GetUserAgentFromType(user_agent_type):
+  """Returns a user agent string based on a user agent type.
   This is derived from:
   https://developers.google.com/chrome/mobile/docs/user-agent
   """
   if user_agent_type:
-    return ['--user-agent=%s' % UA_TYPE_MAPPING[user_agent_type]]
-  return []
+    return UA_TYPE_MAPPING[user_agent_type]
+  return None