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 <jonasolsson@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27786}
diff --git a/rtc_base/experiments/field_trial_parser.h b/rtc_base/experiments/field_trial_parser.h
index a63cb20..73b6ca8 100644
--- a/rtc_base/experiments/field_trial_parser.h
+++ b/rtc_base/experiments/field_trial_parser.h
@@ -38,6 +38,7 @@
 class FieldTrialParameterInterface {
  public:
   virtual ~FieldTrialParameterInterface();
+  std::string key() const { return key_; }
 
  protected:
   // Protected to allow implementations to provide assignment and copy.
@@ -55,9 +56,8 @@
 
   std::vector<FieldTrialParameterInterface*> sub_parameters_;
 
-  std::string key_;
-
  private:
+  std::string key_;
   bool used_ = false;
 };