RTC: Avoid reusing web feature id

kRtcPeerConnectionId is 2384
2383 is already taken by kRTCConstraintEnableDtlsSrtpFalse

tools/metrics/histograms/enums.xml was already correct
but web_feature.mojom needing correcting.

No release branches affected:
M66 Branched Chromium @ revision: 540276
Bug was introduced: Cr-Commit-Position: refs/heads/master@{#540486}

BUG=775415
TBR=eladalon@chromium.org

Change-Id: I4baccfe7ce21cc24ca704fbc1b49e92ae3365a80
Reviewed-on: https://chromium-review.googlesource.com/954524Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541725}

third_party/WebKit/public/platform/web_feature.mojom

@@ -1874,7 +1874,7 @@ enum WebFeature {
   kV8RTCRtpSender_Dtmf_AttributeGetter = 2381,
   kRTCConstraintEnableDtlsSrtpTrue = 2382,
   kRTCConstraintEnableDtlsSrtpFalse = 2383,
-  kRtcPeerConnectionId = 2383,
+  kRtcPeerConnectionId = 2384,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.