Add 4 Variations.SeedDateSkew.* histograms to analyze whether the system clock or the build time co…

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 e225050bcaa860786c5c188d42f6de4ebb4f4f8f..7c19e64c0ca7c2c23f710c682ceebeafa99d6709 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -36503,6 +36503,42 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Variations.SeedDateSkew.BuildTimeAheadBy" units="days">
+  <owner>gab@chromium.org</owner>
+  <summary>
+    Reported if the build time is ahead of or within 24 hours of the
+    kVariationsSeedDate. Used as an experiment to see whether the build time
+    could be used to discard very old seeds.

[Alexei Svitkine (slow), 2014/10/02 13:00:33]

Mention when this is logged (Logged on start up when creating field trials from
the variations seed.) - do so for all of these.

[gab, 2014/10/02 14:03:23]

Done.

+  </summary>
+</histogram>
+
+<histogram name="Variations.SeedDateSkew.BuildTimeBehindBy" units="days">
+  <owner>gab@chromium.org</owner>
+  <summary>
+    Reported if the build time is behind the kVariationsSeedDate by a day or
+    more. Used as an experiment to see whether the build time could be used to
+    discard very old seeds.
+  </summary>
+</histogram>
+
+<histogram name="Variations.SeedDateSkew.SystemClockAheadBy" units="days">
+  <owner>gab@chromium.org</owner>
+  <summary>
+    Reported if the system clock is ahead of or within 24 hours of the
+    kVariationsSeedDate. Used as an experiment to see whether the system clock
+    could be used to discard very old seeds.
+  </summary>
+</histogram>
+
+<histogram name="Variations.SeedDateSkew.SystemClockBehindBy" units="days">
+  <owner>gab@chromium.org</owner>
+  <summary>
+    Reported if the system clock is behind the kVariationsSeedDate by a day or
+    more. Used as an experiment to see whether the system clock could be used to
+    discard very old seeds.
+  </summary>
+</histogram>
+
 <histogram name="Variations.SeedEmpty" enum="VariationsSeedEmpty">
   <owner>asvitkine@chromium.org</owner>
   <summary>