[Zucchini] Implement integration test.

This CL adds a new target that runs integration test (implemented with gtest).
It also adds a gclient hook that unconditionally fetches the input data from 
google cloud storage to testdata/

Bug: 729154
Change-Id: I60fb87db94b44b11e687818eb4ffa36918894670
Reviewed-on: https://chromium-review.googlesource.com/619886
Reviewed-by: Dirk Pranke <[email protected]>
Reviewed-by: Erik Chen <[email protected]>
Reviewed-by: Samuel Huang <[email protected]>
Commit-Queue: Etienne Pierre-Doray <[email protected]>
Cr-Commit-Position: refs/heads/master@{#496027}
8 files changed