Avoid NOTREACHED for WebRtcSignalingFrame.

Chromium crash fix when receiving an ack for WebRtcSignalingFrame
in LogSharingMessageAckTime.

Change-Id: I905a5c451501fe862a92801a952774b8c977aa5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066787Reviewed-by: Richard Knoll <knollr@chromium.org>
Auto-Submit: Himanshu Jaju <himanshujaju@chromium.org>
Commit-Queue: Himanshu Jaju <himanshujaju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743509}

chrome/browser/sharing/sharing_metrics.cc

@@ -273,6 +273,7 @@ void LogSharingMessageAckTime(chrome_browser_sharing::MessageType message_type,
       break;
     case chrome_browser_sharing::MessageType::SMS_FETCH_REQUEST:
     case chrome_browser_sharing::MessageType::DISCOVERY_REQUEST:
+    case chrome_browser_sharing::MessageType::WEB_RTC_SIGNALING_FRAME:
       base::UmaHistogramCustomTimes(
           type_suffixed_name, time,
           /*min=*/base::TimeDelta::FromMilliseconds(1),