Don't ignore GetProcessTimes failures

ProcessMetrics silently ignores failures in GetProcessTimes because of
concerns about race conditions, but since ProcessMetrics *owns* a
duplicated process handle such concerns are not valid. If they were then
we could get occasional handle errors when debugging or using
Application Verifier which is unacceptable any. So, adding a DCHECK.

Change-Id: I97475f4223ab71be87ae9025d3f8a00fe2363ff0
Bug: 890051
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601540Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658393}

base/process/process_metrics_win.cc

@@ -143,9 +143,9 @@ TimeDelta ProcessMetrics::GetCumulativeCPUUsage() {
 
   if (!GetProcessTimes(process_.Get(), &creation_time, &exit_time, &kernel_time,
                        &user_time)) {
-    // We don't assert here because in some cases (such as in the Task Manager)
-    // we may call this function on a process that has just exited but we have
-    // not yet received the notification.
+    // This should never fail because we duplicate the handle to guarantee it
+    // will remain valid.
+    DCHECK(false);
     return TimeDelta();
   }