Reset old low entropy source when reseting all client ids.

This is triggered by the cloned install detector here:
https://cs.chromium.org/chromium/src/components/metrics/cloned_install_detector.cc?rcl=4c806b8b802c5bcd93831229bedd8164d8a68a26&l=82

Bug: 866722
Change-Id: Ic0c99bdceee10413949f71f965d1b4edb9006466
Reviewed-on: https://chromium-review.googlesource.com/c/1392348Reviewed-by: Paul Miller <paulmiller@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619705}

components/metrics/metrics_state_manager.cc

@@ -423,6 +423,7 @@ void MetricsStateManager::ResetMetricsIDsIfNecessary() {
 
   local_state_->ClearPref(prefs::kMetricsClientID);
   local_state_->ClearPref(prefs::kMetricsLowEntropySource);
+  local_state_->ClearPref(prefs::kMetricsOldLowEntropySource);
   local_state_->ClearPref(prefs::kMetricsResetIds);
 
   // Also clear the backed up client info.