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..7f6db2196c413b03104a511d5c717a3a6a214ca5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35013,6 +35013,27 @@ 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>
+    It records how many requests of the given type the user performed each day
+    where the type is specified by the _suffix of the histogram. The histogram
+    is tracked only after midnight of the given day passes - right before the

[Marc Treib, 2016/07/20 10:19:06]

"tracked" isn't quite right - the values are always tracked, it's *emitted*
after midnight

[jkrcal, 2016/07/20 12:09:28]

Done.

+    first following request (which can be several days later if the user does
+    not use chrome in the meantime). Forced requests are not counted in this
+    histogram.
+  </summary>
+</histogram>
+
+<histogram name="NewTabPage.RequestCounter.Total"

[Marc Treib, 2016/07/20 10:19:06]

Can we replace "Total" by something more meaningful - "RequestStatus" maybe?

[jkrcal, 2016/07/20 12:09:28]

Done.

+    enum="NtpRequestCounterStatus">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Records all requests of the given type and their status w.r.t. the quota
+    imposed. The type of request is specified by the _suffix of the histogram.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.SearchURLs.Total">
   <obsolete>
     Deprecated 2016-02 (and not recorded for some time before that).
@@ -84792,6 +84813,12 @@ 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 +99938,12 @@ 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.Total"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="RequestMediaKeySystemAccessKeySystems" separator=".">
   <suffix name="ClearKey" label="Requests for the Clear Key key system."/>
   <suffix name="Unknown"