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This CL introduces a way of defining a JSON config file as a "base JSON
config file" + "delta config file".
The generated config follows the following rules:
- If a key is present in both the "base config" and the "delta config" the
  value in the "delta config" is used.
- If a key is present in one of the "base config" and the "delta config"
  the key+value are included in the generated config.

The reason for this change are to:
1) Make it easier to add test configs
2) Avoid breakages due to the configs being out of sync such as
https://chromium-review.googlesource.com/c/chromium/src/+/1298428

BUG=899274

Change-Id: I48f236c4d6a86cba32c144b773ba6aab8c99b783
Reviewed-on: https://chromium-review.googlesource.com/c/1302326
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603282}