Custom Tabs: add speculation status histograms.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 6ed192b3a430a9c42cd098e58111a7082c27aa59..253409b482bbf03822d34f864edfe0e6a4acbd41 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9958,6 +9958,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="CustomTabs.SpeculationStatusOnStart"
+    enum="CustomTabsSpeculationStatusOnStart">
+  <owner>mattcary@chromium.org</owner>
+  <summary>
+    Recorded only for Android. How a speculation was started or why it was
+    disallowed. This is recorded when a speculation of any kind is requested,
+    and, if allowed, also when the specific kind of speculation is started.

[Mark P, 2017/05/30 22:09:54]

The semantics for this seem weird.  Can only one type of speculation be started?
 When do you record "allowed but not started" (upon request, as the description
implies)?  Will it be started?  When are aborted recorded?  At the same time
(upon request) or later?  If at the same time, then why don't you record
"requested but not allowed" as opposed "started and aborted"?

Please rethink this a bit and get back to me.

[mattcary, 2017/05/31 08:15:40]

I agree that the semantics are a bit weird. There are general reasons why
speculation can't be done at all (all the "aborted" statuses). For prerender,
there are additional edge cases where prerender is not started that are only
discovered when the prerender is actually created (see
CustomTabsConnection.prerenderUrl).

[Mark P, 2017/06/01 05:19:35]

You answered some of my question but not all, and did not revise the description
at all.  I understand this is complicated, which is why it's important to write
a good description while this is all clear in your mind so your future self and
your coworkers can remember the nuances here.  In particular, please have the
description answer:
* Can only one type of speculation be started?
* When do you record "allowed but not started" (upon request, as the description
implies)?  Will it be started?
* When are aborted recorded?  At the same time (upon request) or later or
earlier?
   * If at the same time, then why don't you record "requested but not allowed"
as opposed "started and aborted"?
  * IF earlier, can it be recorded without a corresponding "started" call?
* Does every abort call have an (emitted) started call associated with it?

Also, more broadly (answer me, not in the histograms.xml description) is it
useful to have a single histogram that lumps together aborts for different kinds
of speculations without any way to sort out how common each is for each?  I'd
think separate aborted histograms would make sense for distinct types of
speculations...

[mattcary, 2017/06/06 14:39:23]

Updated, LMK.

The reason why these values appear in the same histogram is that their values
are only useful relative to all counts: for example, abort counts or start
counts relative to total number of speculations allowed. Just a, for example,
"abort" histogram isn't informative or actionable by itself. It seems less
error-prone to have this information in a single histogram, then to expect users
to always query for a particular set of histograms.

[Mark P, 2017/06/06 18:25:50]

Thanks.  Left some comments on the latest patchset.
Understood about the usefulness of putting all this in one histogram.

A thought for the future: perhaps it would be useful to have separate "abort"
histograms for the different types of speculations.  Maybe the types of requests
that trigger, say, prerender speculation might be prohibited for a different set
of reasons (more often device type, for example) than the types of requests that
trigger prefetch speculations.  Maybe those are more often blocked due to, say,
data reduction enabled.

[mattcary, 2017/06/07 11:55:47]

That's a good point. For now, the abort is decided independently of the
speculation type.

+  </summary>
+</histogram>
+
+<histogram name="CustomTabs.SpeculationStatusOnSwap"
+    enum="CustomTabsSpeculationStatusOnSwap">
+  <owner>mattcary@chromium.org</owner>
+  <summary>
+    Recorded only for Android. Recorded when a speculation is requesting to swap
+    into a visible tab.

[Mark P, 2017/05/30 22:09:54]

Is this true, that the browser speculation decides when to swap-in, not the
browser itself?  Is a speculation always eligible to be swapped in?  (E.g., what
if the user switched tabs; what does a speculation request to swap-in get
recorded as?).

[mattcary, 2017/05/31 08:15:40]

Both. CustomTabs decides to launch a URL, and the current speculation, if any,
decides if that URL is a match for it. A speculation is currently only eligible
to be swapped in if the URL matches; if it doesn't match we record the "not
matched" histograms.

+  </summary>
+</histogram>
+
 <histogram name="CustomTabs.Visible" enum="VisibleTab">
   <owner>yusufo@chromium.org</owner>
   <summary>