Add Precache.Fetch.MinWeight UMA.

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 5aaf095bcbadf0f60c8deb5cb1597140eac99975..7f20018558ae6d0365f3f0e2ccb394b626e1a84f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -50632,6 +50632,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Precache.Fetch.MinWeight" units="thousandths">

[rkaplow, 2017/02/23 04:02:14]

kind of an odd unit - should it be thousandth of resource weight? What is weight
actually here - some internal metric?

[twifkak, 2017/02/23 23:28:42]

Yes it's thousandth of resource weight (since we don't support real-valued
histos, AFAICT). Resource weight is a funny internal thing, and its meaning
depends on the value of resource_weight_function in the experiment config:

1. If FUNCTION_NAIVE, then it's the expected number of page visits in the next
30 days needing that resource.
2. If NUNCTION_GEOMETRIC, then it's the probability of a page visit in the next
30 days needing that resource.

I can rename the unit if you like, but I assumed it needed to be something
shortish for the UMA UI.

+  <owner>twifkak@chromium.org</owner>
+  <summary>
+    The minimum resource weight that is fetched in a given precache run.
+    Reported if the precache completed successfully or was cancelled due to the
+    byte cap, but not if it was cancelled due to an ill-formed manifest or
+    exceeding the time limit. Reported only if global_ranking is true.
+  </summary>
+</histogram>
+
 <histogram name="Precache.Fetch.PercentCompleted" units="%">
   <owner>twifkak@chromium.org</owner>
   <owner>bengr@chromium.org</owner>