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..b4fc7baac46a9be32bae2cc01d3fc04d520c5451 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

[Marc Treib, 2016/07/20 12:58:27]

nit: "the user" kinda sounds like these are all user-initiated requests

[Marc Treib, 2016/07/20 12:58:27]

nitty nit: Remove the leading "It"

[jkrcal, 2016/07/20 14:20:17]

Done.

[jkrcal, 2016/07/20 14:20:17]

Done.

+    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). Forced requests are not counted in this

[Marc Treib, 2016/07/20 12:58:27]

nitty nit 2: capitalize Chrome

[jkrcal, 2016/07/20 14:20:17]

Done.

+    histogram.

[Marc Treib, 2016/07/20 12:58:27]

...but requests that weren't actually made, because the quota was exhausted,
*are* counted, which is a bit surprising and at least worth a mention here.

[jkrcal, 2016/07/20 14:20:17]

Maybe surprising but I think it gives the best insight into the behaviour of the
throttled fetcher. Comment changed.

+  </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.
+  </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.RequestStatus"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="RequestMediaKeySystemAccessKeySystems" separator=".">
   <suffix name="ClearKey" label="Requests for the Clear Key key system."/>
   <suffix name="Unknown"