CT Google Storage utils to download/upload artifacts for workers.

BUG=skia:3163
TBR=jcgreogrio

Review URL: https://codereview.chromium.org/779633003
Add create_page_set worker_script

The create pageset worker script reads in Alexa's top1M CSV and creates pagesets
for the worker. These pagesets are cached locally and uploaded to Google Storage.

This script calls task and timestamp utilities from https://codereview.chromium.org/776073002/ and GS utilities from https://codereview.chromium.org/779633003/

BUG=skia:3163

Review URL: https://codereview.chromium.org/773263002