Benchmark: `--save-all-traces` argument

Benchmark: Replace `--save-trace` argument by `--save-all-traces`. It outputs a .trace file per benchmark as before, but stores all traces, without filtering by the measurements defined in the benchmarks file. fixes #458

BUG=
R=etiennej@chromium.org, ppi@chromium.org, qsr@chromium.org

Review URL: https://codereview.chromium.org/1391013005 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: ebcc63fd341a027acd15b38924779bd63b9bc9a2

mojo_benchmark

diff --git a/mojo_benchmark b/mojo_benchmark
index fc0258e..8f83c5b 100755
--- a/mojo_benchmark
+++ b/mojo_benchmark
@@ -148,7 +148,7 @@
       description=_DESCRIPTION)
   parser.add_argument('benchmark_list_file', type=file,
                       help='a file listing benchmarks to run')
-  parser.add_argument('--save-traces', action='store_true',
+  parser.add_argument('--save-all-traces', action='store_true',
                       help='save the traces produced by benchmarks to disk')
   perf_dashboard.add_argparse_server_arguments(parser)
 
@@ -179,7 +179,7 @@
       measurements = variant_spec['measurements']
 
       output_file = None
-      if script_args.save_traces:
+      if script_args.save_all_traces:
         output_file = 'benchmark-%s-%s-%s.trace' % (
             benchmark_name.replace(' ', '_'),
             variant_name.replace(' ', '_'),