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This class uses the Builder pattern to write the config file loaded by
ExternalConstantsOverrider. "Builder" is not common in C++, compared to
Java, but here it seems like the easiest way to express our knowledge
about what fields are supported in this file type; it will create
reasonably terse integration test code and can be extended as new fields
are added to the set of overridable constants.

Bug: 1154901
Change-Id: Ia17280e5a6eaad8cd4e1d1f04f5fd0515ac2e3c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631185
Commit-Queue: Adam Norberg <norberg@google.com>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844892}