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This CL introduces media::Tuneable, which creates a tuneable parameter
based on a finch field trial parameter.  It caches the value to avoid
redundant string conversions, and also has handy helpers for types
that aren't supported by field trial params (base::TimeDelta).

Change-Id: I8b1a6ec3ab0d7f5e36f8ebd496ae2c44e8cd414b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2164227Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792391}