Revert "Removes string support in field trial parser."

This reverts commit e74156f7d05cf3c9858e554789b3f4bb3b93cc19.

Reason for revert: This turned out to be useful :)

Original change's description:
> Removes string support in field trial parser.
> 
> This prepares for simplifying the behavior of optionals so that
> an empty parameter value resets the optional.
> 
> Bug: webrtc:9883
> Change-Id: I8ef8fe9698235044cac66bc4a587abe874c8f854
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150883
> Commit-Queue: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#29061}

TBR=terelius@webrtc.org,srte@webrtc.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: webrtc:9883
Change-Id: Idbb4061f4b423987e62f3a9ad9bee2410e2cec96
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152383
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29137}