Remove become_active DCHECK as it would fail on warm start.

The DCHECK would fail if re-open the app.  Remove the DCHECK.
Warm start would be handled in a separate CL.

Bug: 
Change-Id: I0db26e976322b8e0600a7251757c8bfbc31e4c89
Reviewed-on: https://chromium-review.googlesource.com/572005
Commit-Queue: Menglu Huang <huangml@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486837}

ios/testing/perf/startupLoggers.mm

@@ -28,7 +28,6 @@ void RegisterAppDidFinishLaunchingTime() {
 
 void RegisterAppDidBecomeActiveTime() {
   DCHECK(g_start_time);
-  DCHECK(!g_become_active_time);
   g_become_active_time = new base::Time(base::Time::Now());
 }