Fix potential memory leak in WifiServiceMac.

The Windows counterpart to WifiServiceMac calls UnIninitialize in the
destructor. It appears that the Mac version is failing to call Uninitialize,
which may be the source of a large notification-related memory leak.

Bug: 904540
Change-Id: I718aa58d69f4f39450bbb78f05375750ce4d6735
Reviewed-on: https://chromium-review.googlesource.com/c/1342088Reviewed-by: Misha Efimov <mef@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612711}

components/wifi/wifi_service_mac.mm

@@ -159,6 +159,7 @@ WiFiServiceMac::WiFiServiceMac() : wlan_observer_(nil) {
 }
 
 WiFiServiceMac::~WiFiServiceMac() {
+  UnInitialize();
 }
 
 void WiFiServiceMac::Initialize(