The first call to gsutil causes a network call and makes the download
script slow also in the most optimal cases. This CL refactors the
download script and moves the first gsutil call after checking locally
if sha1s match.

1) This turns the input acquisition into a generator and buffers the
files and sha1s in a list before multithreading.

2) This sequentially checks the sha1s and files and bails out early if
all match. In Chrome-land, we usually call this script with only one
file. There are some cases with around 4. This could also be
parallelized if the need arises.

3) The initial gsutil check, which ensures gsutil is updated, is moved
right in front of the multithreaded downloads.

The performance of one call to download_from_google_storage for an
existing 500MB file is 2.3s before this CL and 1.2s after (most of the
remaining time left is spent for making sha1sum).

Example for full gclient runhooks (when everything is up-to-date):

Chromium: 32s before, 12s after
V8: 12s before, 3s after

Bug: 776311
Change-Id: Ia7715a6af84b1b336455ea88494d399bdb050317
Reviewed-on: https://chromium-review.googlesource.com/897562
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>