[Stats] Migrate RTCStats/Test to Attributes.

One in series of CLs to migrate from Members() to Attributes() to
unblock code deletion.

Adds and makes use of has_value(), holds_alternative<T>() and get<T>()
convenience methods.

Bug: webrtc:15164
Change-Id: I2ae9869a5d1de3f3875c70494c9781d69da7f936
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/334160
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@google.com>
Cr-Commit-Position: refs/heads/main@{#41510}

stats/attribute.cc

diff --git a/stats/attribute.cc b/stats/attribute.cc
index 36cdc454..b2e2986 100644
--- a/stats/attribute.cc
+++ b/stats/attribute.cc
@@ -99,6 +99,11 @@
   return attribute_;
 }
 
+bool Attribute::has_value() const {
+  return absl::visit([](const auto* attr) { return attr->has_value(); },
+                     attribute_);
+}
+
 RTCStatsMemberInterface::Type Attribute::type() const {
   return absl::visit([](const auto* attr) { return attr->type(); }, attribute_);
 }