Introduce a request throttler for limiting requests in mobile NTP.

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 8bf9229d7609cd263b2133725dd613cbd4ec15c6..111f4251c0ba1753e0ccd5f86cba96f679874e52 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35013,6 +35013,28 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.RequestCounter.PerDay" units="requests">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Records how many requests of the given type the browser tried to perform
+    each day where the type is specified by the _suffix of the histogram. The
+    histogram is emitted only after midnight of the given day passes - right
+    before the first following request (which can be several days later if the
+    user does not use Chrome in the meantime). The histogram counts requests
+    with both QUOTA_GRANTED and QUOTA_EXCEEDED status, i.e. the count can easily
+    exceed the daily quota. Forced requests are not counted here.
+  </summary>
+</histogram>
+
+<histogram name="NewTabPage.RequestCounter.RequestStatus"
+    enum="NtpRequestCounterStatus">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Records for all requests of the given type their status w.r.t. the quota
+    imposed. The type of request is specified by the _suffix of the histogram.

[Marc Treib, 2016/07/20 15:13:58]

Grammar nit: "Records the status w.r.t. the quota for all requests of the given
type."

[jkrcal, 2016/07/20 15:46:23]

Done.

+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.SearchURLs.Total">
   <obsolete>
     Deprecated 2016-02 (and not recorded for some time before that).
@@ -50702,7 +50724,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
     Whether search results were seen, whether the search provider icon sprite
     was animated when the panel first appeared, and the triggering gesture. If
     animation is disabled due to a field trial, we still log
-    &quot;animated&quot; if the animation would have run otherwise.
+    &quot;animated&quot;if the animation would have run otherwise.

[Marc Treib, 2016/07/20 15:13:58]

Unintentional?

[jkrcal, 2016/07/20 15:46:23]

Indeed!

   </summary>
 </histogram>
 
@@ -84792,6 +84814,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="2" label="NTP Promo link clicked"/>
 </enum>
 
+<enum name="NtpRequestCounterStatus" type="int">
+  <int value="0" label="Forced request"/>
+  <int value="1" label="Quota granted"/>
+  <int value="2" label="Quota exceeded"/>
+</enum>
+
 <enum name="NtpSnippetsFetchResult" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Empty hosts"/>
@@ -99911,6 +99939,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Event.Latency.RendererImpl"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="RequestCounterTypes">
+  <suffix name="Fetcher" label="Fetcher for content suggestions on mobile NTP"/>
+  <affected-histogram name="NewTabPage.RequestCounter.PerDay"/>
+  <affected-histogram name="NewTabPage.RequestCounter.RequestStatus"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="RequestMediaKeySystemAccessKeySystems" separator=".">
   <suffix name="ClearKey" label="Requests for the Clear Key key system."/>
   <suffix name="Unknown"