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 8a7a577ecfb3f6eb9d33bda98f934a4bf496115d..59f87b4b22fca58ea7ddbb56c76976d80c21493e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -50990,6 +50990,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Precache.Fetch.MinWeight" units="thousandths">
+  <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 canceled 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>