Avoid setting NetworkChangeNotifierFactory multiple times for AwBrowserContextTest

Setup() is called on each test and it will set the global
NetworkChangeNotifierFactory multiple times, causing a CHECK
failure in NetworkChangeNotifier::SetFactory().

This CL fixes the issue by not setting the factory if it is
already set.

BUG=1023050

Change-Id: I61e2ca89e0c2409bdd8f8725c2e96bf706618da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1909810Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714606}

android_webview/browser/aw_browser_context_unittest.cc

@@ -17,6 +17,12 @@ namespace android_webview {
 
 class AwBrowserContextTest : public testing::Test {
  protected:
+  // Runs before the first test
+  static void SetUpTestSuite() {
+    net::NetworkChangeNotifier::SetFactory(
+        new AwNetworkChangeNotifierFactory());
+  }
+
   void SetUp() override {
     mojo::core::Init();
     test_content_client_initializer_ =
@@ -25,8 +31,6 @@ class AwBrowserContextTest : public testing::Test {
     AwFeatureListCreator* aw_feature_list_creator = new AwFeatureListCreator();
     aw_feature_list_creator->CreateLocalState();
     browser_process_ = new AwBrowserProcess(aw_feature_list_creator);
-    net::NetworkChangeNotifier::SetFactory(
-        new AwNetworkChangeNotifierFactory());
   }
 
   void TearDown() override {