is there more work here that still needs to be done, BIll? if purely UMA, we should look to getting the merge back done soon.

Handing this bug over to Anna for the new UKMs she is adding.  Existing ones are in, and the ones that aren't in 66 aren't worth risk for merging back.

(give back to me if you won't be able to get these in, but it seems natural for you to work on them as you add new UKMs)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/70560c29cf695c2c7bbfc4b7726e937ebd6ed887

commit 70560c29cf695c2c7bbfc4b7726e937ebd6ed887
Author: Anna Offenwanger <offenwanger@chromium.org>
Date: Wed Apr 04 19:07:41 2018

Add UMA for XR PageSession StartAction

Adding in a UMA that logs what triggered the entry into Chrome in VR.

Bug:  808589 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I6bc9e9501fd86428e37db53853fc638fa00c1e73
Reviewed-on: https://chromium-review.googlesource.com/989055
Reviewed-by: Yash Malik <ymalik@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548150}
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellDelegate.java
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/browser/android/vr/vr_shell.cc
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/browser/android/vr/vr_shell.h
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/browser/android/vr/vr_shell_delegate.cc
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/browser/android/vr/vr_shell_delegate.h
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/browser/vr/metrics/session_metrics_helper.cc
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/chrome/browser/vr/metrics/session_metrics_helper.h
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/70560c29cf695c2c7bbfc4b7726e937ebd6ed887/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/661598c35bfa2f20edd1c7983d5f5d223ca80aab

commit 661598c35bfa2f20edd1c7983d5f5d223ca80aab
Author: Anna Offenwanger <offenwanger@chromium.org>
Date: Tue Apr 10 22:12:37 2018

Add XR Presentation Start Action UMA

Adding a histogram to track how the user got into XR Presentation.

Bug:  808589 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Iad597f74fe22afac5709ebee1525c0b342cc24a4
Reviewed-on: https://chromium-review.googlesource.com/999019
Commit-Queue: Anna Offenwanger <offenwanger@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549658}
[modify] https://crrev.com/661598c35bfa2f20edd1c7983d5f5d223ca80aab/chrome/browser/vr/metrics/session_metrics_helper.cc
[modify] https://crrev.com/661598c35bfa2f20edd1c7983d5f5d223ca80aab/chrome/browser/vr/metrics/session_metrics_helper.h
[modify] https://crrev.com/661598c35bfa2f20edd1c7983d5f5d223ca80aab/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/661598c35bfa2f20edd1c7983d5f5d223ca80aab/tools/metrics/histograms/histograms.xml