Add histogram Omnibox.FocusToEditTime for time in milliseconds between a user focus and edit.

chrome/browser/ui/omnibox/omnibox_edit_model.cc

Index: chrome/browser/ui/omnibox/omnibox_edit_model.cc
===================================================================
--- chrome/browser/ui/omnibox/omnibox_edit_model.cc	(revision 209999)
+++ chrome/browser/ui/omnibox/omnibox_edit_model.cc	(working copy)
@@ -93,6 +93,10 @@
 // in the EnteredKeywordModeMethod enum which is defined in the .h file.
 const char kEnteredKeywordModeHistogram[] = "Omnibox.EnteredKeywordMode";
 
+// Histogram name which counts the number of milliseconds a user takes
+// between focusing and editing the omnibox.
+const char kFocusToEditHistogram[] = "Omnibox.FocusToEdit";
+
 }  // namespace
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -407,6 +411,16 @@
 }
 
 void OmniboxEditModel::SetInputInProgress(bool in_progress) {
+  if (in_progress && !last_omnibox_focus_without_user_input_.is_null()) {
+    base::TimeTicks now = base::TimeTicks::Now();
+    DCHECK(last_omnibox_focus_without_user_input_ <= now);

[Peter Kasting, 2013/07/09 00:43:21]

Nit: DCHECK_LE

However, I don't think this is guaranteed.  I don't think Now() has any
guarantee of being monotonically increasing -- cases like DST starting could
cause it to jump backwards.

You may want to verify this and possibly replace this DCHECK with a conditional
+ comment.

[H Fung, 2013/07/09 01:09:55]

Actually, it looks like TimeTicks is guaranteed to not decrease:
https://code.google.com/p/chromium/codesearch#chromium/src/base/time/time.h&l=16

I tried to use DCHECK_LE but got a compiler error.  I did not dig into why
DCHECK_LE only works for certain types, though.  (Calling ToInternalValue()
works with DCHECK_LE, but I think that code would be worse).

[Peter Kasting, 2013/07/09 01:21:46]

Oh good.  I must have been thinking of Time::Now().
That means there's no overloaded "print to string" method on TimeTicks that the
macros can use to print things out.  In theory we could define one, but what
you're doing is fine.

+    UMA_HISTOGRAM_TIMES(kFocusToEditHistogram,
+                        now - last_omnibox_focus_without_user_input_);
+    // We only want to count the time from focus to the first user input, so
+    // reset |last_omnibox_focus_without_user_input_|.
+    last_omnibox_focus_without_user_input_ = base::TimeTicks();
+  }
+
   if (user_input_in_progress_ == in_progress)
     return;
 
@@ -802,6 +816,8 @@
 }
 
 void OmniboxEditModel::OnSetFocus(bool control_down) {
+  last_omnibox_focus_without_user_input_ = base::TimeTicks::Now();
+
   // If the omnibox lost focus while the caret was hidden and then regained
   // focus, OnSetFocus() is called and should restore visibility. Note that
   // focus can be regained without an accompanying call to