Add --download-data-only option to run-test.py

The simd.js benchmarks reside in the same repository
that holds the compliance tests.

Adding an option to run-test.py to download test data
and exit, so that a recipe to run the simd.js
benchmarks can call run-test.py with this option
to fetch the test data.

BUG=https://code.google.com/p/v8/issues/detail?id=4124
LOG=N
TEST=manual verification it works.
NOTRY=true
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1140353002

Cr-Commit-Position: refs/heads/master@{#28446}

tools/run-tests.py

@@ -206,6 +206,9 @@ def BuildOptions():
                     default="")
   result.add_option("--download-data", help="Download missing test suite data",
                     default=False, action="store_true")
+  result.add_option("--download-data-only",
+                    help="Download missing test suite data and exit",
+                    default=False, action="store_true")
   result.add_option("--extra-flags",
                     help="Additional flags to pass to each test command",
                     default="")
@@ -485,10 +488,13 @@ def Main():
     if suite:
       suites.append(suite)
 
-  if options.download_data:
+  if options.download_data or options.download_data_only:
     for s in suites:
       s.DownloadData()
 
+  if options.download_data_only:
+    return exit_code
+
   for (arch, mode) in options.arch_and_mode:
     try:
       code = Execute(arch, mode, args, options, suites, workspace)