Teach `mojo_benchmark` to clear network service disk cache.

R=qsr@chromium.org

Review URL: https://codereview.chromium.org/1345753002 .

mojo/devtools/common/mojo_benchmark

diff --git a/mojo/devtools/common/mojo_benchmark b/mojo/devtools/common/mojo_benchmark
index 5373d35..12a80d7 100755
--- a/mojo/devtools/common/mojo_benchmark
+++ b/mojo/devtools/common/mojo_benchmark
@@ -45,6 +45,7 @@
 
 _BENCHMARK_APP = 'https://core.mojoapps.io/benchmark.mojo'
 _CACHE_SERVICE_URL = 'mojo:url_response_disk_cache'
+_NETWORK_SERVICE_URL = 'mojo:network_service'
 
 # Additional time in seconds allocated per shell run to accommodate start-up.
 # The shell should terminate before hitting this time out, it is an error if it
@@ -78,6 +79,7 @@
 
   if cold_start:
     shell_args.append('--args-for=%s %s' % (_CACHE_SERVICE_URL, '--clear'))
+    shell_args.append('--args-for=%s %s' % (_NETWORK_SERVICE_URL, '--clear'))
 
   if verbose:
     print 'shell arguments: ' + str(shell_args)