Teach mojo_benchmark to control caching conditions.

This patch makes mojo_benchmark run each benchmark twice, first with a
flushed cache and again without clearing the cache, yielding cold start
and warm start results.

This currently supports only flushing the url_response_disk_cache, we
will want to clear the network service cache too.

R=qsr@google.com

Review URL: https://codereview.chromium.org/1320403003 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 152015f44e134512ba45524e562c7c547b28886d

mojo_benchmark

diff --git a/mojo_benchmark b/mojo_benchmark
index 7de9e61..756b44b 100755
--- a/mojo_benchmark
+++ b/mojo_benchmark
@@ -43,6 +43,7 @@
 _logger = logging.getLogger()
 
 _BENCHMARK_APP = 'https://core.mojoapps.io/benchmark.mojo'
+_CACHE_SERVICE_URL = 'mojo:url_response_disk_cache'
 
 # 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
@@ -51,8 +52,8 @@
 
 
 def _run_benchmark(shell, shell_args, name, app, duration_seconds, measurements,
-                   verbose):
-  """Runs `benchmark.mojo` in a shell with correct arguments, parses and
+                   cold_start, verbose):
+  """Runs `benchmark.mojo` in shell with correct arguments, parses and
   presents the benchmark results.
   """
   timeout = duration_seconds + _EXTRA_TIMEOUT
@@ -61,12 +62,18 @@
   benchmark_args.append('--duration=' + str(duration_seconds))
   for measurement in measurements:
     benchmark_args.append(measurement)
+
+  shell_args = list(shell_args)
   shell_args.append(_BENCHMARK_APP)
   shell_args.append('--args-for=%s %s' % (_BENCHMARK_APP,
                                           ' '.join(benchmark_args)))
+
+  if cold_start:
+    shell_args.append('--args-for=%s %s' % (_CACHE_SERVICE_URL, '--clear'))
+
   if verbose:
     print 'shell arguments: ' + str(shell_args)
-  print '[' + name + ']'
+  print '[ %s ] %s' % (name, 'cold start' if cold_start else 'warm start')
   return_code, output, did_time_out = shell.run_and_get_output(
       shell_args, timeout=timeout)
   output_lines = [line.strip() for line in output.split('\n')]
@@ -118,7 +125,9 @@
     shell_args = benchmark_spec.get('shell-args', []) + common_shell_args
     measurements = benchmark_spec['measurements']
     _run_benchmark(shell, shell_args, name, app, duration, measurements,
-                   script_args.verbose)
+                   cold_start=True, verbose=script_args.verbose)
+    _run_benchmark(shell, shell_args, name, app, duration, measurements,
+                   cold_start=False, verbose=script_args.verbose)
 
   return 0 if succeeded else 1