[Offline indicator v2] Add force online flag to startup.mobile

This CL follows up on crrev.com/c/2358509 to add the command-line flag
to startup.mobile benchmarks.

Bug: 1122611
Change-Id: I3dacb532da3a11a98209dff1d8ecdb13cdecfb28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386538Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Sinan Sahin <sinansahin@google.com>
Cr-Commit-Position: refs/heads/master@{#803313}

tools/perf/benchmarks/startup_mobile.py

@@ -307,3 +307,10 @@ class MobileStartupBenchmark(perf_benchmark.PerfBenchmark):
   @classmethod
   def Name(cls):
     return 'startup.mobile'
+
+  def SetExtraBrowserOptions(self, options):
+    super(MobileStartupBenchmark, self).SetExtraBrowserOptions(options)
+    # Force online state for the offline indicator so it doesn't show and affect
+    # the benchmarks on bots, which are offline by default.
+    options.AppendExtraBrowserArgs(
+        '--force-online-connection-state-for-indicator')