Add a mechanism for collecting Rappor samples on a daily interval.

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 ce6325a46fa61b12806f60dc300e2f5b404b4a8b..a6c68e9a800f00f925ef357d03a280ea08aed3a4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3350,6 +3350,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="DailyInterval.IntervalType" enum="DailyIntervalIntervalType">
+  <owner>holte@chromium.org</owner>
+  <summary>Counts how often daily interval events were fired.</summary>
+</histogram>
+
 <histogram name="DataReductionProxy.BlockTypeFallback"
     enum="DataReductionProxyBypassType">
   <owner>bengr@chromium.org</owner>
@@ -38461,6 +38466,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="8" label="PlayMusic"/>
 </enum>
 
+<enum name="DailyIntervalIntervalType" type="int">
+  <int value="0" label="First Run"/>
+  <int value="1" label="Day Elapsed"/>
+  <int value="2" label="Clock Changed"/>
+</enum>
+
 <enum name="DataChannelCounters" type="int">
   <int value="0" label="Channel created."/>
   <int value="1" label="Channel reached Open state."/>
@@ -51455,6 +51466,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="CrosFirstRun.TimeSpentOnStep"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="DailyInterval" separator=".">
+  <suffix name="rappor.last_daily_sample"
+      label="Daily intervals for rappor metric collection"/>
+  <affected-histogram name="DailyInterval.IntervalType"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="DataReductionProxy">
   <suffix name="DataReductionProxy"
       label="Only page loads through the data reduction proxy are considered."/>