DevTools: Add UMA for Viewing trace from Audits

Bug: 880555
Change-Id: I997fc1e8665c1cdee38c626bf6652fb5e792651c
Reviewed-on: https://chromium-review.googlesource.com/1205374
Commit-Queue: Paul Irish <paulirish@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588752}

third_party/WebKit/LayoutTests/http/tests/devtools/user-metrics-expected.txt

@@ -5,6 +5,7 @@ recordActionTaken:
     AnimationsPlaybackRateChanged : 11
     Audits2Finished : 30
     Audits2Started : 29
+    Audits2ViewTrace : 32
     AuditsStarted : 7
     ChangeInspectedNodeInElementsPanel : 26
     CommandEvaluatedInConsolePanel : 15

third_party/blink/renderer/devtools/front_end/audits2/Audits2ReportRenderer.js

@@ -20,6 +20,7 @@ Audits2.ReportRenderer = class extends ReportRenderer {
     return el;
 
     async function onViewTraceClick() {
+      Host.userMetrics.actionTaken(Host.UserMetrics.Action.Audits2ViewTrace);
       await UI.inspectorView.showPanel('timeline');
       Timeline.TimelinePanel.instance().loadFromEvents(defaultPassTrace.traceEvents);
     }

third_party/blink/renderer/devtools/front_end/host/UserMetrics.js

@@ -94,6 +94,7 @@ Host.UserMetrics.Action = {
   Audits2Started: 29,
   Audits2Finished: 30,
   ShowedThirdPartyBadges: 31,
+  Audits2ViewTrace: 32,
 };
 
 Host.UserMetrics._PanelCodes = {

tools/metrics/histograms/enums.xml

@@ -10331,6 +10331,7 @@ Called by update_net_error_codes.py.-->
   <int value="29" label="Audits started"/>
   <int value="30" label="Audits finished"/>
   <int value="31" label="Showed third party badges"/>
+  <int value="32" label="Audits' view trace clicked"/>
 </enum>
 
 <enum name="DevToolsPanel">