update_engine: Merge remote-tracking branch 'cros/upstream' into cros/master

Since libchrome in AOSP is ahead of CrOS I had to guard against BASE_VER in a
few places to satisfy older libchromes.

file_fetcher.cc is now needed in delta_generator.

A few unittests need to be run as root.

BUG=chromium:916593
TEST=unittest
TEST=cros_generate_update_payload
TEST=cros flash
CQ-DEPEND=CL:1399261

Change-Id: If3497549e88e559f8ecc38f414259b9c774f4a44