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

tools/telemetry/telemetry/browser_backend.py

Index: tools/telemetry/telemetry/browser_backend.py
diff --git a/tools/telemetry/telemetry/browser_backend.py b/tools/telemetry/telemetry/browser_backend.py
index de9e0929bb618401f0e7f3604d303952a279296d..09855bf33b97721a6a6a8870e52a3416bdf9790b 100644
--- a/tools/telemetry/telemetry/browser_backend.py
+++ b/tools/telemetry/telemetry/browser_backend.py
@@ -13,7 +13,6 @@ from telemetry import browser_gone_exception
 from telemetry import options_for_unittests
 from telemetry import tab_list_backend
 from telemetry import tracing_backend
-from telemetry import user_agent
 from telemetry import util
 from telemetry import wpr_modes
 from telemetry import wpr_server
@@ -67,8 +66,6 @@ class BrowserBackend(object):
       args.extend(wpr_server.GetChromeFlags(self.WEBPAGEREPLAY_HOST,
                                             self.WEBPAGEREPLAY_HTTP_PORT,
                                             self.WEBPAGEREPLAY_HTTPS_PORT))
-    args.extend(user_agent.GetChromeUserAgentArgumentFromType(
-        self.options.browser_user_agent_type))
     return args
 
   @property