[Zucchini] Download integration test data using multiple threads.

As a follow-up to https://chromium-review.googlesource.com/619886:
It was found that this adds ~30s to download test files on a fresh
Chromium install. This CL alleviates the problem by fetching files
(4 x ~1 MB each) using multiple threads.

Bug: 729154
Change-Id: I7e85d3b3321979882a41db7d761aacf360aff5ea
Reviewed-on: https://chromium-review.googlesource.com/650830
Commit-Queue: Samuel Huang <[email protected]>
Reviewed-by: Dirk Pranke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#499757}
1 file changed