[Extensions] Add UMA for extensions client initialization time

It seems that ChromeExtensionsClient::Init() is taking a non-trivial
amount of time (see linked bug). Add UMA so that we can track it,
and measure any future improvements.

Bug: 847237
Change-Id: I1b86dd9a8f2a7500841f253580a9e6d64c9f4d45
Reviewed-on: https://chromium-review.googlesource.com/1142617Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577700}

chrome/common/extensions/chrome_extensions_client.cc

@@ -105,6 +105,8 @@ ChromeExtensionsClient::~ChromeExtensionsClient() {
 }
 
 void ChromeExtensionsClient::Initialize() {
+  SCOPED_UMA_HISTOGRAM_TIMER("Extensions.ChromeExtensionsClientInitTime");
+
   // Registration could already be finalized in unit tests, where the utility
   // thread runs in-process.
   if (!ManifestHandler::IsRegistrationFinalized()) {

tools/metrics/histograms/histograms.xml

@@ -27200,6 +27200,17 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Extensions.ChromeExtensionsClientInitTime" units="ms"
+    expires_after="2019-12-01">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <owner>extensions-core@chromium.org</owner>
+  <summary>
+    The amount of elapsed time taken to initialize the ChromeExtensionsClient.
+    Recorded once per client initialization, which happens once per instance of
+    Chrome.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.ContentVerification.ComputedHashesInitTime"
     units="ms" expires_after="2019-11-30">
   <owner>lazyboy@chromium.org</owner>