Adds getter for FieldTrialParameter keys.
Adds getter for FieldTrialParameter keys.

This is useful in test tooling.

Bug: webrtc:9346
Change-Id: I4a2ac52927cfe72f392f8748d3bada1e88db1b6a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/134209
Reviewed-by: Jonas Olsson <7906496040a7ac5328c0e08cb0fb50d1a12c27fd@webrtc.org>
Commit-Queue: Sebastian Jansson <c24d1bc144eb7c11e553f59153fa17dba85ca83e@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27786}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc