Bug 1429510 - Make last shutdown time a scalar. r=chutten

toolkit/components/telemetry/Scalars.yaml

@@ -1311,6 +1311,22 @@ timestamps:
     record_in_processes:
       - main
 
+browser.timings:
+  last_shutdown:
+    bug_numbers:
+      - 1429510
+    description: >
+      The time, in milliseconds, it took to complete the last shutdown.
+      On successful shutdown, Telemetry saves this to disk into Telemetry.ShutdownTime.txt.
+      On the next startup this is loaded and recorded.
+    expires: never
+    kind: uint
+    notification_emails:
+      - florian@mozilla.com
+    release_channel_collection: opt-in
+    record_in_processes:
+      - 'main'
+
 # The following section contains the service worker scalars.
 sw:
   synthesized_res_count:

toolkit/components/telemetry/Telemetry.cpp

@@ -312,6 +312,10 @@ public:
     mTelemetry->mLastShutdownTime =
       ReadLastShutdownDuration(mShutdownTimeFilename);
     mTelemetry->ReadLateWritesStacks(mProfileDir);
+
+    TelemetryScalar::Set(Telemetry::ScalarID::BROWSER_TIMINGS_LAST_SHUTDOWN,
+                         mTelemetry->mLastShutdownTime);
+
     nsCOMPtr<nsIRunnable> e =
       NewRunnableMethod("nsFetchTelemetryData::MainThread",
                         this,

toolkit/components/telemetry/tests/unit/test_TelemetrySession.js

@@ -330,6 +330,10 @@ function checkPayload(payload, reason, successfulPings, savedPings) {
   let activeTicks = payload.simpleMeasurements.activeTicks;
   Assert.ok(activeTicks >= 0);
 
+  if ("browser.timings.last_shutdown" in payload.processes.parent.scalars) {
+    Assert.equal(payload.processes.parent.scalars["browser.timings.last_shutdown"], SHUTDOWN_TIME);
+  }
+
   Assert.equal(payload.simpleMeasurements.failedProfileLockCount,
               FAILED_PROFILE_LOCK_ATTEMPTS);
   let profileDirectory = Services.dirsvc.get("ProfD", Ci.nsIFile);