The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8

commit 65b5d3f839ffd1efa0c7fab2e12e762b773a35d8
Author: David Trainor <dtrainor@chromium.org>
Date: Tue Jan 16 18:13:22 2018

Add TaskLoadedNative step to BackgroundTaskScheduler

Add a metric for when the NativeBackgroundTask actually starts up the
native library.  Since NativeBackgroundTask does live in the same
package as the UMA code, added an external UMA class that is publicly
accessible that exposes public-facing UMA methods.

BUG= 800494 

Change-Id: I2df44d0d7fc746189eb4e2485e512d6d47c1879f
Reviewed-on: https://chromium-review.googlesource.com/857331
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529462}
[modify] https://crrev.com/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8/chrome/android/java/src/org/chromium/chrome/browser/background_task_scheduler/NativeBackgroundTask.java
[modify] https://crrev.com/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8/components/background_task_scheduler/BUILD.gn
[add] https://crrev.com/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskSchedulerExternalUma.java
[modify] https://crrev.com/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8/components/background_task_scheduler/android/java/src/org/chromium/components/background_task_scheduler/BackgroundTaskSchedulerUma.java
[modify] https://crrev.com/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8/components/background_task_scheduler/android/junit/src/org/chromium/components/background_task_scheduler/BackgroundTaskSchedulerUmaTest.java
[modify] https://crrev.com/65b5d3f839ffd1efa0c7fab2e12e762b773a35d8/tools/metrics/histograms/histograms.xml