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

tools/telemetry/telemetry/inspector_page.py

Index: tools/telemetry/telemetry/inspector_page.py
diff --git a/tools/telemetry/telemetry/inspector_page.py b/tools/telemetry/telemetry/inspector_page.py
index 3551e37ff4918a3e142e0e1f412abd1e13bd31d9..b96a45b1614dba9930309832f45edb80278616ad 100644
--- a/tools/telemetry/telemetry/inspector_page.py
+++ b/tools/telemetry/telemetry/inspector_page.py
@@ -62,6 +62,20 @@ class InspectorPage(object):
 
     DisablePageNotifications()
 
+  def SetUserAgent(self, user_agent):
+    request = {
+        'method': 'Network.enable'
+        }
+    self._tab_backend.SyncRequest(request)
+
+    request = {
+        'method': 'Network.setUserAgentOverride',
+        'params': {
+            'userAgent': user_agent
+            }
+        }
+    self._tab_backend.SyncRequest(request)
+
   def Navigate(self, url, timeout=60):
     """Navigates to url"""