Revert of Remove Enable Exits On Suspend flags on iOS

ios/chrome/app/UIApplication+ExitsOnSuspend.mm

+// Copyright 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#if !defined(NDEBUG)
+
+#import "ios/chrome/app/UIApplication+ExitsOnSuspend.h"
+
+#include "base/ios/block_types.h"
+#include "base/logging.h"
+#include "base/threading/thread_restrictions.h"
+
+NSString* const kExitsOnSuspend = @"EnableExitsOnSuspend";
+
+namespace {
+int backgroundTasksCount = 0;
+UIBackgroundTaskIdentifier countTaskIdentifier = UIBackgroundTaskInvalid;
+
+// Perform a block on the main thread. Asynchronously if the thread is not the
+// main thread, synchronously if the thread is the main thread.
+void ExecuteBlockOnMainThread(ProceduralBlock block) {
+  if ([NSThread isMainThread])
+    block();
+  else
+    dispatch_async(dispatch_get_main_queue(), block);
+}
+}  // namespace
+
+// Category defining interposing methods. These methods keep a tally of the
+// background tasks count.
+@implementation UIApplication (BackgroundTasksCounter)
+
+// Method to replace -beginBackgroundTaskWithExpirationHandler:. The original
+// method is called within.
+- (UIBackgroundTaskIdentifier)
+    cr_interpose_beginBackgroundTaskWithExpirationHandler:
+        (ProceduralBlock)handler {
+  UIBackgroundTaskIdentifier identifier =
+      [self cr_interpose_beginBackgroundTaskWithExpirationHandler:handler];
+  if (identifier != UIBackgroundTaskInvalid) {
+    ExecuteBlockOnMainThread(^{
+      backgroundTasksCount++;
+    });
+  }
+  return identifier;
+}
+
+// Method to replace -endBackgroundTask:. The original method is called within.
+- (void)cr_interpose_endBackgroundTask:(UIBackgroundTaskIdentifier)identifier {
+  if (identifier != UIBackgroundTaskInvalid)
+    ExecuteBlockOnMainThread(^{
+      backgroundTasksCount--;
+    });
+  [self cr_interpose_endBackgroundTask:identifier];
+}
+
+@end
+
+@interface UIApplication (ExitsOnSuspend_Private)
+// Terminate the app immediately. exit(0) is used.
+- (void)cr_terminateImmediately;
+// Terminate the app via -cr_terminateImmediately when the background tasks
+// count is one. The remaining task is the count observation task.
+- (void)cr_terminateWhenCountIsOne;
+@end
+
+@implementation UIApplication (ExitsOnSuspend)
+
+- (void)cr_terminateWhenDoneWithBackgroundTasks {
+  // Add a background task for the count observation.
+  DCHECK(countTaskIdentifier == UIBackgroundTaskInvalid);
+  countTaskIdentifier = [self beginBackgroundTaskWithExpirationHandler:^{
+    // If we get to the end of the 10 minutes, exit.
+    [self cr_terminateImmediately];
+  }];
+
+  [self cr_terminateWhenCountIsOne];
+}
+
+- (void)cr_cancelTermination {
+  [NSObject
+      cancelPreviousPerformRequestsWithTarget:self
+                                     selector:@selector(
+                                                  cr_terminateWhenCountIsOne)
+                                       object:nil];
+
+  // Cancel the count observation background task.
+  [self endBackgroundTask:countTaskIdentifier];
+  countTaskIdentifier = UIBackgroundTaskInvalid;
+}
+
+#pragma mark - Private
+
+- (void)cr_terminateImmediately {
+  DVLOG(1) << "App exited when suspended after running background tasks.";
+  // exit(0) will trigger at_exit handlers. Some need to be run on a IOAllowed
+  // thread, such as file_util::MemoryMappedFile::CloseHandles().
+  base::ThreadRestrictions::SetIOAllowed(true);
+  exit(0);
+}
+
+- (void)cr_terminateWhenCountIsOne {
+  if (backgroundTasksCount <= 1)
+    [self cr_terminateImmediately];
+  [self performSelector:_cmd withObject:nil afterDelay:1];
+}
+
+@end
+
+#endif  // !defined(NDEBUG)