Implement RTC[In/Out]boundRtpStreamStats.contentType.

Spec: https://henbos.github.io/webrtc-provisional-stats/#dom-rtcinboundrtpstreamstats-contenttype

This already exists as a goog-stat. This CL only plumbs the value to the
new stats collector.

Note: There is currently no distinction between the extension being
missing and it being present but the value being "unspecified". Until
https://crbug.com/webrtc/10529 is fixed, this metric is only exposed if
SCREENSHARE was present.

Bug: webrtc:10452
Change-Id: Ic8723f4d0efb43ab72a560e954676facd3b90659
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131946
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27520}
diff --git a/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index 13eb213..174e78e 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -740,8 +740,12 @@
     if (inbound_stream.media_type.is_defined() &&
         *inbound_stream.media_type == "video") {
       verifier.TestMemberIsDefined(inbound_stream.frames_decoded);
+      // The integration test is not set up to test screen share; don't require
+      // this to be present.
+      verifier.MarkMemberTested(inbound_stream.content_type, true);
     } else {
       verifier.TestMemberIsUndefined(inbound_stream.frames_decoded);
+      verifier.TestMemberIsUndefined(inbound_stream.content_type);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }
@@ -764,9 +768,13 @@
       verifier.TestMemberIsDefined(outbound_stream.frames_encoded);
       verifier.TestMemberIsNonNegative<double>(
           outbound_stream.total_encode_time);
+      // The integration test is not set up to test screen share; don't require
+      // this to be present.
+      verifier.MarkMemberTested(outbound_stream.content_type, true);
     } else {
       verifier.TestMemberIsUndefined(outbound_stream.frames_encoded);
       verifier.TestMemberIsUndefined(outbound_stream.total_encode_time);
+      verifier.TestMemberIsUndefined(outbound_stream.content_type);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }