Implement certificate chain stats.

There was an implementation, but it relied on SSLCertificate::GetChain,
which was never implemented. Except in the fake certificate classes
used by the stats collector tests, hence the tests were passing.

Instead of implementing GetChain, we decided (in
https://webrtc-review.googlesource.com/c/src/+/6500) to add
methods that return a SSLCertChain directly, since it results in a
somewhat cleaner object model.

So this CL switches everything to use the "chain" methods, and gets
rid of the obsolete methods and member variables.

Bug: webrtc:8920
Change-Id: Ie9d7d53654ba859535462521b54c788adec7badf
Reviewed-on: https://webrtc-review.googlesource.com/56961
Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Zhi Huang <zhihuang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22177}
diff --git a/pc/test/fakepeerconnectionbase.h b/pc/test/fakepeerconnectionbase.h
index 99964cb..5ca1ba7 100644
--- a/pc/test/fakepeerconnectionbase.h
+++ b/pc/test/fakepeerconnectionbase.h
@@ -292,7 +292,7 @@
     return false;
   }
 
-  std::unique_ptr<rtc::SSLCertificate> GetRemoteSSLCertificate(
+  std::unique_ptr<rtc::SSLCertChain> GetRemoteSSLCertChain(
       const std::string& transport_name) override {
     return nullptr;
   }