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 00cc5237a7e93259c93b64e92cbc57ee8e86bb38..f3f62054b3e831739316cde83aa8a3f152b7a243 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35036,6 +35036,28 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.RequestThrottler.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.RequestThrottler.RequestStatus"
+    enum="NtpRequestThrottlerStatus">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Records the status w.r.t. the quota for all requests of the given type. 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).
@@ -84953,6 +84975,12 @@ 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="NtpRequestThrottlerStatus" 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"/>
@@ -100099,6 +100127,13 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Media.EME.RequestMediaKeySystemAccess"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="RequestThrottlerTypes">
+  <suffix name="SuggestionFetcher"
+      label="Fetcher for content suggestions on mobile NTP"/>
+  <affected-histogram name="NewTabPage.RequestThrottler.PerDay"/>
+  <affected-histogram name="NewTabPage.RequestThrottler.RequestStatus"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="ResourceLoaderSizeSlice" separator=".">
   <suffix name="LT_2kB" label="Sliced for resources smaller than 2kB."/>
   <suffix name="LT_32kB"