Should we record the result for fullscreen requests and the corresponding reason (EnumerationHistogram) instead of UseCounter?

I think it would be better to have a number to compare with. I have the following in mind:
A) Fullscreen request allowed by user gesture.
B) Fullscreen request allowed by orientation change.
C) Fullscreen request blocked.

Another way is to have two UseCounters for A) and B) respectively.

WDYT?

In our specific case, I think B is fine. We want to know whether the feature we added is used. The benefit from using a UseCounter is that we know the information "per document" instead of every time it happens so I would prefer to have a UseCounter for A and B instead of an histogram.

+foolip@ to see if he is interested in a UseCounter for A.

Measuring just B would be fine with me.

(There are plenty of things that go wrong before actually being in fullscreen, so if we suspected that FullscreenSecureOrigin was overcounting and not representative of the real risk to something that happens after entering fullscreen, then maybe a use counter in Fullscreen::pushFullscreenElementStack would be useful.)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/12268f9ed6ab22159f1b22ba9af1a6a5ec3faec0

commit 12268f9ed6ab22159f1b22ba9af1a6a5ec3faec0
Author: zqzhang <zqzhang@chromium.org>
Date: Wed Dec 07 13:51:19 2016

[Blink>Fullscreen] Add UseCounter for fullscreen on orientation change

This CL adds UseCounter to record the number of requests for
fullscreen on device orientation change.

BUG= 671223 

Review-Url: https://codereview.chromium.org/2557443002
Cr-Commit-Position: refs/heads/master@{#436935}

[modify] https://crrev.com/12268f9ed6ab22159f1b22ba9af1a6a5ec3faec0/third_party/WebKit/Source/core/dom/Fullscreen.cpp
[modify] https://crrev.com/12268f9ed6ab22159f1b22ba9af1a6a5ec3faec0/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/12268f9ed6ab22159f1b22ba9af1a6a5ec3faec0/tools/metrics/histograms/histograms.xml

Your change meets the bar and is auto-approved for M56 (branch: 2924)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/eb670ebd58eaa60a612192ce6f0f80aec0038eb5

commit eb670ebd58eaa60a612192ce6f0f80aec0038eb5
Author: Zhiqiang Zhang <zqzhang@google.com>
Date: Thu Dec 08 15:50:57 2016

[Blink>Fullscreen] Add UseCounter for fullscreen on orientation change

This CL adds UseCounter to record the number of requests for
fullscreen on device orientation change.

BUG= 671223 

Review-Url: https://codereview.chromium.org/2557443002
Cr-Commit-Position: refs/heads/master@{#436935}
(cherry picked from commit 12268f9ed6ab22159f1b22ba9af1a6a5ec3faec0)

Review URL: https://codereview.chromium.org/2564573002 .

Cr-Commit-Position: refs/branch-heads/2924@{#404}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}

[modify] https://crrev.com/eb670ebd58eaa60a612192ce6f0f80aec0038eb5/third_party/WebKit/Source/core/dom/Fullscreen.cpp
[modify] https://crrev.com/eb670ebd58eaa60a612192ce6f0f80aec0038eb5/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/eb670ebd58eaa60a612192ce6f0f80aec0038eb5/tools/metrics/histograms/histograms.xml