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

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

Committed: https://crrev.com/f3a18ee6626fd838cf141b2364f350ecec79fb03
Cr-Commit-Position: refs/heads/master@{#28446}

[Michael Achenbach, 2015-05-18 14:03:17]

LGTM - I've set LOG=n in the cl desc as we only add api changes and the like to
our change log.

[Michael Achenbach, 2015-05-18 14:04:16]

I've also set NOTRY...

[bradn, 2015-05-18 14:09:36]

The CQ bit was checked by bradnelson@google.com

[commit-bot: I haz the power, 2015-05-18 14:09:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1140353002/1

[commit-bot: I haz the power, 2015-05-18 14:11:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 14:11:37]

Patchset 1 (id:??) landed as
https://crrev.com/f3a18ee6626fd838cf141b2364f350ecec79fb03
Cr-Commit-Position: refs/heads/master@{#28446}

[Michael Achenbach, 2015-05-18 14:18:34]

FYI: It should work to write: BUG=v8:4124

Then bugdroid automatically adds the CL to the bug.