Removing  issue 678367  as being blocked by this as having these metrics is in fact not a blocker for having user interaction triggers implemented.

But I still want to have this (simple work) done before the potential M58 launch of last_n.

Making the title more generic as we're interested in more than efficiency only: impact, usefulness, benefits, cost, etc.

We noticed we need some new metrics to more accurately assess the impact of last_n on tab restores:
1. # of tab restores.
2. # of tab restores that happened when user is offline.
3. # of tab restores where we actually showed a last_n saved offline page.

Have been working on this since last week.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e9f7809b63666b9c8c558dbde9397b4ce23fe119

commit e9f7809b63666b9c8c558dbde9397b4ce23fe119
Author: carlosk <carlosk@chromium.org>
Date: Tue Mar 21 20:21:02 2017

Adds metric to help assess last_n impact on tab restores.

This change adds a new metric to help us assess the impact of last_n on
the cases where an existing tab has to have its contents restored. This
change only affects Chrome on Android.

We track tab restores in Tab and call the appropriate, newly introduced
methods in OfflinePageUtils where the reporting actually happens.

BUG= 688588 

Review-Url: https://codereview.chromium.org/2754103003
Cr-Commit-Position: refs/heads/master@{#458530}

[modify] https://crrev.com/e9f7809b63666b9c8c558dbde9397b4ce23fe119/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java
[modify] https://crrev.com/e9f7809b63666b9c8c558dbde9397b4ce23fe119/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
[modify] https://crrev.com/e9f7809b63666b9c8c558dbde9397b4ce23fe119/chrome/android/java/src/org/chromium/chrome/browser/tab/TabWebContentsObserver.java
[modify] https://crrev.com/e9f7809b63666b9c8c558dbde9397b4ce23fe119/tools/metrics/histograms/histograms.xml

Your change meets the bar and is auto-approved for M58. Please go ahead and merge the CL to branch 3029 manually. Please contact milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), bhthompson@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/79e7e4dafbc6b93344e3f71e872baafd80f7ca9e

commit 79e7e4dafbc6b93344e3f71e872baafd80f7ca9e
Author: Carlos Knippschild <carlosk@chromium.org>
Date: Tue Mar 21 23:06:51 2017

Adds metric to help assess last_n impact on tab restores.

This change adds a new metric to help us assess the impact of last_n on
the cases where an existing tab has to have its contents restored. This
change only affects Chrome on Android.

We track tab restores in Tab and call the appropriate, newly introduced
methods in OfflinePageUtils where the reporting actually happens.

BUG= 688588 

Review-Url: https://codereview.chromium.org/2754103003
Cr-Commit-Position: refs/heads/master@{#458530}
(cherry picked from commit e9f7809b63666b9c8c558dbde9397b4ce23fe119)

Review-Url: https://codereview.chromium.org/2767803002 .
Cr-Commit-Position: refs/branch-heads/3029@{#348}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}

[modify] https://crrev.com/79e7e4dafbc6b93344e3f71e872baafd80f7ca9e/chrome/android/java/src/org/chromium/chrome/browser/offlinepages/OfflinePageUtils.java
[modify] https://crrev.com/79e7e4dafbc6b93344e3f71e872baafd80f7ca9e/chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java
[modify] https://crrev.com/79e7e4dafbc6b93344e3f71e872baafd80f7ca9e/chrome/android/java/src/org/chromium/chrome/browser/tab/TabWebContentsObserver.java
[modify] https://crrev.com/79e7e4dafbc6b93344e3f71e872baafd80f7ca9e/tools/metrics/histograms/histograms.xml

Closing this as the suggested metric was implemented and no other suggestions were made.