AW: fix crash in AwBrowserContextTest

No change to production logic.

This fixes a crash in AwBrowserContextTest. The issue was the second
test in the suite (regardless of what that test did) would crash as soon
as the setup constructs a TestContentClientInitializer. This is because
a posted task created a NetworkConnectionTracker, running after
TestContentClientInitializer's destructor nulled it out. It's invalid to
re-set a NetworkConnectionTracker without first clearing it, triggering
a DCHECK.

We avoid this by pumping through the UI thread looper during the test
teardown, so we're sure the background task has executed prior to
destroying TestContentClientInitializer.

This also fixes a memory leak in the test setup/teardown (we need to
clean up the AwBrowserProcess instance), although this was unrelated
to the bug.

Bug: 990547
Test: run_android_webview_unittests --num_retries=0 --gtest_filter=AwBrowserContextTest.*
Change-Id: I16d03a1cf6d813b7052b8205d2936864397fc21e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849174
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704282}

android_webview/browser/aw_browser_context_unittest.cc

@@ -5,6 +5,7 @@
 #include "android_webview/browser/aw_browser_context.h"
 #include "android_webview/browser/aw_browser_process.h"
 #include "android_webview/browser/aw_feature_list_creator.h"
+#include "base/run_loop.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/test/browser_task_environment.h"
 #include "content/public/test/test_content_client_initializer.h"
@@ -25,7 +26,15 @@ class AwBrowserContextTest : public testing::Test {
     browser_process_ = new AwBrowserProcess(aw_feature_list_creator);
   }
 
-  void TearDown() override { delete test_content_client_initializer_; }
+  void TearDown() override {
+    // Drain the message queue before destroying
+    // |test_content_client_initializer_|, otherwise a posted task may call
+    // content::GetNetworkConnectionTracker() after
+    // TestContentClientInitializer's destructor sets it to null.
+    base::RunLoop().RunUntilIdle();
+    delete test_content_client_initializer_;
+    delete browser_process_;
+  }
 
   // Create the TestBrowserThreads.
   content::BrowserTaskEnvironment task_environment_;