Add disabled-by-default webrtc trace category

Will be used to trace detailed events such as the sending of RTP packets.
See https://webrtc-review.googlesource.com/c/src/+/176231

Bug: webrtc:11617
Change-Id: I3c0d17190805cf39e72e5615af7c4cc3482d8602
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2219497
Commit-Queue: Johannes Kron <kron@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772704}

base/trace_event/builtin_categories.h

@@ -246,6 +246,7 @@
   X(TRACE_DISABLED_BY_DEFAULT("viz.surface_lifetime"))                   \
   X(TRACE_DISABLED_BY_DEFAULT("viz.triangles"))                          \
   X(TRACE_DISABLED_BY_DEFAULT("webaudio.audionode"))                     \
+  X(TRACE_DISABLED_BY_DEFAULT("webrtc"))                                 \
   X(TRACE_DISABLED_BY_DEFAULT("worker.scheduler"))
 
 #define INTERNAL_TRACE_INIT_CATEGORY_NAME(name) name,