UIDevTools: Add UMA metrics to record usage

Use user action metric UI_DevTools_Connect to record each time a
connection to UI DevTools server is established. The frequency of
such connections would help us gauge the usage of the UI DevTools.

BUG=746562

Change-Id: Icd54da4f23b3cc0268113de48931a803af53632c
Reviewed-on: https://chromium-review.googlesource.com/c/1299007
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603629}

components/ui_devtools/devtools_server.cc

@@ -11,6 +11,8 @@
 #include "base/command_line.h"
 #include "base/format_macros.h"
 #include "base/message_loop/message_loop.h"
+#include "base/metrics/user_metrics.h"
+#include "base/metrics/user_metrics_action.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "base/values.h"
@@ -156,7 +158,7 @@ void UiDevToolsServer::MakeServer(
 
 // HttpServer::Delegate Implementation
 void UiDevToolsServer::OnConnect(int connection_id) {
-  NOTIMPLEMENTED();
+  base::RecordAction(base::UserMetricsAction("UI_DevTools_Connect"));
 }
 
 void UiDevToolsServer::OnHttpRequest(

tools/metrics/actions/actions.xml

@@ -19858,6 +19858,14 @@ should be able to be added at any place in this file.
   </description>
 </action>
 
+<action name="UI_DevTools_Connect">
+  <owner>weili@chromium.org</owner>
+  <description>
+    Records when a user connects to a UI DevTools HTTP server, i.e., by clicking
+    on the 'inspect' link on chrome://inspect/#other.
+  </description>
+</action>
+
 <action name="Underline">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <description>Please enter the description of this user action.</description>