Allow AppSearchProvider to provide recommendations.

chrome/browser/ui/app_list/search/app_result.cc

Index: chrome/browser/ui/app_list/search/app_result.cc
diff --git a/chrome/browser/ui/app_list/search/app_result.cc b/chrome/browser/ui/app_list/search/app_result.cc
index 5a966f17957f828e97f19b3d4e402901404ccf4d..60c0b4e797c09315a84a19d11c888a96c76a529c 100644
--- a/chrome/browser/ui/app_list/search/app_result.cc
+++ b/chrome/browser/ui/app_list/search/app_result.cc
@@ -4,6 +4,7 @@
 
 #include "chrome/browser/ui/app_list/search/app_result.h"
 
+#include "base/time/time.h"
 #include "chrome/browser/extensions/extension_service.h"
 #include "chrome/browser/extensions/extension_util.h"
 #include "chrome/browser/profiles/profile.h"
@@ -73,6 +74,16 @@ void AppResult::UpdateFromMatch(const TokenizedString& title,
   set_relevance(match.relevance());
 }
 
+void AppResult::UpdateFromLastLaunched(const base::Time& last_launched) {
+  base::TimeDelta delta = base::Time::Now() - last_launched;
+  DCHECK_LE(0, delta.InSeconds());
+  const int kSecondsInWeek = 60 * 60 * 24 * 7;
+
+  // Set the relevance to a value between 0 and 1. This function decays as the
+  // time delta increases and reaches a value of 0.5 at 1 week.
+  set_relevance(1 / (1 + delta.InSecondsF() / kSecondsInWeek));

[Matt Giuca, 2014/07/22 04:31:07]

Why does the relevance have to be a float? How ingrained is this in the system?
It would be a lot simpler if the relevance could simply be a 64-bit integer.
Then you could just use last_launched.ToInternalValue() as the relevance (higher
is better), and not have to deal with Time::Now(), weeks, floats, or
reciprocals. (See my concerns later in the tests.)

[calamity, 2014/07/22 07:24:04]

Very. Boost values are +1-4 which rely on the base relevance to be 0-1 so all
other providers give relevances in the range 0-1.

[Matt Giuca, 2014/07/23 01:53:18]

Acknowledged.

+}
+
 void AppResult::Open(int event_flags) {
   const extensions::Extension* extension =
       extensions::ExtensionSystem::Get(profile_)->extension_service()