[gaia] Introduce Gaia URLs config

Currently, there are 5 different URL switches in
google_apis/gaia/gaia_switches.cc that need to be manually set to set
non-prod Gaia environment. This CL introduces a new command line switch
to rule them all:

--gaia-config=gaia_test.json

A JSON config file allows to override all URLs available in
google_apis/gaia/gaia_urls.h. For a format example take a look at
google_apis/test/data/gaia/all_urls.json.

The proposed design was chosen because of the following:
- It allows to override each individual URL separately without bloating
the command line. This can be useful for developers who run their own
test instances.
- It makes easy to hide test URLs used at Google in the internal repo.

Bug: 1072731
Change-Id: I3c32c53f9dc27b01ceab8ef57eb71c00d5df0977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166197
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#771605}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d84591a6c37765e9c76e82a68d85a64cf86a4606
14 files changed