[ObjC ARC] Converts ios/chrome/app:main to ARC.

ios/chrome/app/chrome_exe_main.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.
 
 #import <UIKit/UIKit.h>
 
 #include "base/at_exit.h"
 #include "base/debug/crash_logging.h"
 #include "components/crash/core/common/crash_keys.h"
 #include "ios/chrome/app/startup/ios_chrome_main.h"
 #include "ios/chrome/browser/crash_report/breakpad_helper.h"
 #include "ios/chrome/browser/crash_report/crash_keys.h"
 #include "ios/chrome/common/channel_info.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 NSString* const kUIApplicationDelegateInfoKey = @"UIApplicationDelegate";
 
 void StartCrashController() {
   std::string channel_string = GetChannelString();
 
   RegisterChromeIOSCrashKeys();
   base::debug::SetCrashKeyValue(crash_keys::kChannel, channel_string);
   breakpad_helper::Start(channel_string);
 }
 
 }  // namespace
 
 int main(int argc, char* argv[]) {
   IOSChromeMain::InitStartTime();
-  NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
 
   NSUserDefaults* standardDefaults = [NSUserDefaults standardUserDefaults];
 
   // Set NSUserDefaults keys to force pseudo-RTL if needed.
   if ([standardDefaults boolForKey:@"EnablePseudoRTL"]) {
     NSDictionary* pseudoDict = @{ @"YES" : @"AppleTextDirection" };
     [standardDefaults registerDefaults:pseudoDict];
   }
 
   // Create this here since it's needed to start the crash handler.
   base::AtExitManager at_exit;
 
   // The Crash Controller is started here even if the user opted out since we
   // don't have yet preferences. Later on it is stopped if the user opted out.
   // In any case reports are not sent if the user opted out.
   StartCrashController();
 
   // Always ignore SIGPIPE.  We check the return value of write().
   CHECK_NE(SIG_ERR, signal(SIGPIPE, SIG_IGN));
 
-  // Purging the pool to prevent autorelease objects created by the previous
-  // calls to live forever.

[rohitrao (ping after 24h), 2017/05/18 20:50:34]

We added this autorelease pool for a specific reason, so I don't think we should
remove it.  Does it work to add two @autoreleasepool blocks to replace the old
pools?

+sdefresne and +stk to weigh in on this as well, since it's nontrivial.

[stkhapugin, 2017/05/30 14:04:06]

This should be replaced with @autoreleasepool in a separate CL, since
@autoreleasepool macro should build fine without ARC. Please, submit a separate
parent CL that puts @autoreleasepool {} around lines 31-54 and 55-64. 

If this CL lands cleanly, this CL will be trivial. 

[liaoyuke, 2017/06/14 00:02:23]

Have uploaded this parent CL: https://chromium-review.googlesource.com/c/534874/

-  [pool release];
-  pool = [[NSAutoreleasePool alloc] init];
 
   // Part of code that requires us to specify which UIApplication delegate class
   // to use by adding "UIApplicationDelegate" key to Info.plist file.
   NSString* delegateClassName = [[NSBundle mainBundle]
       objectForInfoDictionaryKey:kUIApplicationDelegateInfoKey];
   CHECK(delegateClassName);
 
   int retVal = UIApplicationMain(argc, argv, nil, delegateClassName);
-  [pool release];
   return retVal;
 }