Add functionality for automatic rolling of individual DEPS
entries. This will make it possible to move away from the
links created by setup_links.py to real DEPS entries.
There are two kinds of such deps we intend to use:
1. Third party dependencies used by both WebRTC and Chromium:
those are rolled to the same revision as the Chromium DEPS file of the revision passed
to the script
2. Chromium sub-directories needed for WebRTC (mainly for //build and BUILD.gn
files of third_party deps): those are rolled to the HEAD revision
Notice that the latter kind could be rolled ahead of the chromium_revision,
but generally these should be close (and if it passes the bots, we don't really mind).
The new functionality can coexist with the old one, to
enable a smooth transition (not everything needs to change at once).
Some of the updating logic was inspired by
https://cs.chromium.org/chromium/build/scripts/slave/recipes/v8/auto_roll_v8_deps.py
Add extensive tests for the logic of figuring out deps changes,
including mocking git ls-remote call to keep the tests hermetic.
BUG=webrtc:5006
NOTRY=True
Review-Url: https://codereview.webrtc.org/2570603003
Cr-Commit-Position: refs/heads/master@{#15570}
38b6dbc793