wpt: add pointer about webrtc/h264 failures on buildbots

as buildbots missing H264 comes up frequently.

BUG=chromium:840659

Change-Id: I8db4bee8bbbc04d1227d957dcd6a73ee658ec829
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414315Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#807850}

third_party/blink/web_tests/external/wpt/webrtc/RTCRtpTransceiver-setCodecPreferences.html

@@ -29,6 +29,11 @@
           codecs does not fulfill these requirements, the user agent MUST throw
           an InvalidModificationError.
    */
+  /*
+   * Chromium note: this requires build bots with H264 support. See
+   *   https://bugs.chromium.org/p/chromium/issues/detail?id=840659
+   * for details on how to enable support.
+   */
 
   test((t) => {
     const pc = new RTCPeerConnection();

third_party/blink/web_tests/external/wpt/webrtc/protocol/video-codecs.https.html

@@ -7,6 +7,11 @@
 <script>
 'use strict';
 
+/*
+ * Chromium note: this requires build bots with H264 support. See
+ *   https://bugs.chromium.org/p/chromium/issues/detail?id=840659
+ * for details on how to enable support.
+ */
 // Tests for conformance to RFC 7742,
 // "WebRTC Video Processing and Codec Requirements"
 // The document was formerly known as draft-ietf-rtcweb-video-codecs.

third_party/blink/web_tests/external/wpt/webrtc/simulcast/h264.https.html

@@ -8,6 +8,11 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 <script>
+/*
+ * Chromium note: this requires build bots with H264 support. See
+ *   https://bugs.chromium.org/p/chromium/issues/detail?id=840659
+ * for details on how to enable support.
+ */
 promise_test(async t => {
   assert_implements('getCapabilities' in RTCRtpSender, 'RTCRtpSender.getCapabilities not supported');
   assert_implements(RTCRtpSender.getCapabilities('video').codecs.find(c => c.mimeType === 'video/H264'), 'H264 not supported');