Stop observing prefs on MemoryDebuggerManager destroy

Bug: 893486
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I23066a159be3b58d4eca39595eae0ce74495ad71
Reviewed-on: https://chromium-review.googlesource.com/c/1270741
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598356}
diff --git a/ios/chrome/browser/memory/memory_debugger_manager.mm b/ios/chrome/browser/memory/memory_debugger_manager.mm
index 5c73865..2dff786 100644
--- a/ios/chrome/browser/memory/memory_debugger_manager.mm
+++ b/ios/chrome/browser/memory/memory_debugger_manager.mm
@@ -65,6 +65,7 @@
 
 // Tears down the debugger so it can be deallocated.
 - (void)tearDownDebugger {
+  showMemoryDebugger_.Destroy();
   [memoryDebugger_ invalidateTimers];
   [memoryDebugger_ removeFromSuperview];
   memoryDebugger_ = nil;