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This CL refactors the field formatter such that non-integer keys are
also supported. This is in preparation for an upcoming CL where this
will be required.

Bug: b/145043394
Change-Id: I603ea66b2ef338d43c263cfa00031eb11dda565d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2235615
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#777327}