The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ce9386ebd62a74afbe15f0fd0f0c10751b14169f

commit ce9386ebd62a74afbe15f0fd0f0c10751b14169f
Author: Olivier Robin <olivierrobin@chromium.org>
Date: Thu Nov 29 09:07:38 2018

[iOS] Detect freeze in main run loop.

This CL adds a watchdog to check the main thread
responsiveness by posting regular tasks to the main
thread and check their execution.

If the main thread is not responsive for too long,
a "crash" report will be generated and sent on next
startup.
A NSUserDefault will also be set to allow reporting
the freeze on next startup.

Bug:  910024 
Change-Id: I541b27c993b90e0982227cbe4238a338938d760f
Reviewed-on: https://chromium-review.googlesource.com/c/1350958
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612111}
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/about_flags.mm
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/crash_report/BUILD.gn
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/crash_report/breakpad_helper.mm
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/crash_report/crash_report_flags.cc
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/crash_report/crash_report_flags.h
[add] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/crash_report/main_thread_freeze_detector.h
[add] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/crash_report/main_thread_freeze_detector.mm
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/ios_chrome_flag_descriptions.cc
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/ios_chrome_flag_descriptions.h
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider.h
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider.mm
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/ios/chrome/browser/metrics/mobile_session_shutdown_metrics_provider_unittest.mm
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/ce9386ebd62a74afbe15f0fd0f0c10751b14169f/tools/metrics/histograms/histograms.xml