Create Memory Helper for memory warnings

ios/chrome/app/application_delegate/memory_warning_helper.mm

+// Copyright 2016 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.
+
+#import "ios/chrome/app/application_delegate/memory_warning_helper.h"
+
+#include "base/memory/memory_pressure_listener.h"
+#include "base/metrics/histogram.h"
+#include "ios/chrome/browser/crash_report/breakpad_helper.h"
+#import "ios/chrome/browser/metrics/previous_session_info.h"
+
+namespace {
+// The number of seconds to wait after a memory warning to clear the flag used
+// to detect Out Of Memory crashes.
+// NOTE: From local tests on various devices, this interval ranges between 1 and
+// 3 seconds. It is set to 5 to ensure all out of memory crashes are identified,
+// even if this may lead to overcounting them.
+const CFTimeInterval kOutOfMemoryResetTimeInterval = 5;
+}
+
+@interface MemoryWarningHelper () {
+  // The time at which to reset the OOM crash flag in the user defaults. This
+  // is used to handle receiving multiple memory warnings in short
+  // succession.
+  CFAbsoluteTime _outOfMemoryResetTime;
+}
+// Resets the OOM crash flag from the user defaults.
+- (void)resetOutOfMemoryFlagIfNecessary;
+@end
+
+@implementation MemoryWarningHelper
+
+@synthesize foregroundMemoryWarningCount = _foregroundMemoryWarningCount;
+
+- (void)handleMemoryPressure {
+  // Notify the system that the memory is critical and something should be done.
+  base::MemoryPressureListener::NotifyMemoryPressure(
+      base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_CRITICAL);
+
+  ++_foregroundMemoryWarningCount;
+  // Register that we might die because of memory. If we are still alive in
+  // |kOutOfMemoryResetTimeInterval| seconds, reset the flag.
+  [[PreviousSessionInfo sharedInstance] setMemoryWarningFlag];
+  _outOfMemoryResetTime =
+      CFAbsoluteTimeGetCurrent() + kOutOfMemoryResetTimeInterval;
+
+  dispatch_after(kOutOfMemoryResetTimeInterval, dispatch_get_main_queue(), ^{
+    [self resetOutOfMemoryFlagIfNecessary];
+  });
+
+  // Add information to breakpad.

[sdefresne, 2016/05/19 10:09:33]

nit: I would move this before the "dispatch_after" as creating a block does
allocate a tiny bit of memory and could cause the application to be killed at
that point.

+  breakpad_helper::SetMemoryWarningCount(_foregroundMemoryWarningCount);
+  breakpad_helper::SetMemoryWarningInProgress(true);
+}
+
+- (void)resetOutOfMemoryFlagIfNecessary {
+  if (CFAbsoluteTimeGetCurrent() < _outOfMemoryResetTime)
+    return;
+  _outOfMemoryResetTime = 0;
+  [[PreviousSessionInfo sharedInstance] resetMemoryWarningFlag];
+  breakpad_helper::SetMemoryWarningInProgress(false);
+}
+
+- (void)resetForegroundMemoryWarningCount {
+  _foregroundMemoryWarningCount = 0;
+}
+
+@end