The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11e831d923b0ae3faa5d199fbcb5b97ae89d9793

commit 11e831d923b0ae3faa5d199fbcb5b97ae89d9793
Author: Min Qin <qinmin@chromium.org>
Date: Tue Aug 22 17:15:39 2017

Record storage permission UMAs for content initiated download

Currently the storage permission UMA only works for context menu downloads.
This CL makes it work for both context menu and content initiated download.
Also, this CL makes it more clear why download is not initiated,
either it is because throttler or storage permission.

BUG=757612

Change-Id: Ib45ac547a84f7494e804e2f32510848c235d0087
Reviewed-on: https://chromium-review.googlesource.com/624935
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496339}
[modify] https://crrev.com/11e831d923b0ae3faa5d199fbcb5b97ae89d9793/chrome/browser/android/download/download_controller.cc
[modify] https://crrev.com/11e831d923b0ae3faa5d199fbcb5b97ae89d9793/chrome/browser/download/download_resource_throttle.cc
[modify] https://crrev.com/11e831d923b0ae3faa5d199fbcb5b97ae89d9793/chrome/browser/download/download_resource_throttle.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d0808447356a75729b383652b796e1d3b6401956

commit d0808447356a75729b383652b796e1d3b6401956
Author: Min Qin <qinmin@chromium.org>
Date: Thu Sep 07 19:13:00 2017

Add new UMA to track target determination failures

Download could silently fail due to failures in target determination.
This CL adds 2 new UMA to track whether target determination completes.
So it allows us to analyze download failure reasons.

BUG=757612

Change-Id: Ie631dc06a97dfc6410f367ffbd06e6bbdb2a497a
Reviewed-on: https://chromium-review.googlesource.com/653179
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#500355}
[modify] https://crrev.com/d0808447356a75729b383652b796e1d3b6401956/content/browser/download/download_item_impl.cc
[modify] https://crrev.com/d0808447356a75729b383652b796e1d3b6401956/content/browser/download/download_stats.h
[modify] https://crrev.com/d0808447356a75729b383652b796e1d3b6401956/tools/metrics/histograms/enums.xml