Add UMA histograms to track very brief or frequent tabs and JS dialogs.

components/app_modal/javascript_app_modal_dialog.cc

Index: components/app_modal/javascript_app_modal_dialog.cc
diff --git a/components/app_modal/javascript_app_modal_dialog.cc b/components/app_modal/javascript_app_modal_dialog.cc
index 8db59363f1730f6a1c01e793aa568045ac5ebcb1..95cfc9190e745e50d489047fd9de3b2e46650654 100644
--- a/components/app_modal/javascript_app_modal_dialog.cc
+++ b/components/app_modal/javascript_app_modal_dialog.cc
@@ -4,6 +4,8 @@
 
 #include "components/app_modal/javascript_app_modal_dialog.h"
 
+#include "base/metrics/histogram_macros.h"
+#include "base/time/time.h"
 #include "build/build_config.h"
 #include "components/app_modal/javascript_dialog_manager.h"
 #include "components/app_modal/javascript_native_dialog_factory.h"
@@ -72,7 +74,8 @@ JavaScriptAppModalDialog::JavaScriptAppModalDialog(
       is_before_unload_dialog_(is_before_unload_dialog),
       is_reload_(is_reload),
       callback_(callback),
-      use_override_prompt_text_(false) {
+      use_override_prompt_text_(false),
+      creation_time_(base::TimeTicks::Now()) {
   EnforceMaxTextSize(message_text, &message_text_);
   EnforceMaxPromptSize(default_prompt_text, &default_prompt_text_);
 }
@@ -94,6 +97,9 @@ void JavaScriptAppModalDialog::Invalidate() {
   if (!IsValid())
     return;
 
+  UMA_HISTOGRAM_FINE_TIMES_100("JSDialogs.FineTiming.DialogLifetime",
+                               base::TimeTicks::Now() - creation_time_);

[Ilya Sherman, 2016/01/26 22:39:04]

Is this intentionally the same histogram name as the one below?  If so, I'd
recommend writing a wrapper function to emit to this histogram, and maybe also
to document why this histogram is recorded in two different places.

[Joe Mason, 2016/01/27 19:43:50]

Done.

+
   AppModalDialog::Invalidate();
   if (!callback_.is_null()) {
     callback_.Run(false, base::string16());
@@ -142,6 +148,9 @@ void JavaScriptAppModalDialog::NotifyDelegate(bool success,
   if (!IsValid())
     return;
 
+  UMA_HISTOGRAM_FINE_TIMES_100("JSDialogs.FineTiming.DialogLifetime",
+                               base::TimeTicks::Now() - creation_time_);

[Ilya Sherman, 2016/01/26 22:39:04]

This histogram is designed to distinguish a dialog that was open for 100
microseconds from one that was open for a millisecond from one that was open for
10 milliseconds.  All of those feel more or less instant to a user.  Could you
please provide me with a bit more context for what you're trying to measure and
understand?

[Joe Mason, 2016/01/27 16:51:21]

An example question I'm trying to answer with this: if users visit a page that
keeps reopening a tab every time they close it, do they keep trying or give up
and leave the tab open in the background? If the former we'd expect to see lots
of events with a short time between closing a tab and opening a tab. But it's
hard to tell before putting the histogram out there and looking at the results
what counts as "short".

Different bad behaviour by pages may result in different times between tabs,
even if they all feel "instant" to the user. The difference between < 1 msec, 1
msec, 2 msec and 10 msec may turn out to be important or it may not. I just
didn't want to prejudge.

[Ilya Sherman, 2016/01/28 05:25:26]

Okay.  I guess it's okay to land these histograms as they are, but I'd encourage
you to plan to come back to them and tweak or remove any histograms that aren't
providing useful data.

+
   if (!callback_.is_null()) {
     callback_.Run(success, user_input);
     callback_.Reset();