webrtc wpt: mark inbound-rtp fecSsrc as not testable

as this requires a flexfec sender which is not available in the
browser.

Bug: None
Change-Id: Ib2afa0f64e6ebd2681a5ae9bed36a8fcb00c9f9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5233482
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1251360}

Author: fippo

webrtc-stats/supported-stats.https.html

@@ -72,6 +72,9 @@
     'outbound-rtp': [
       'rid', // requires simulcast.
     ],
+    'inbound-rtp': [
+      'fecSsrc', // requires FlexFEC to be negotiated.
+    ],
     'media-source': [
       'echoReturnLoss', // requires gUM with an audio input device.
       'echoReturnLossEnhancement', // requires gUM with an audio input device.