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">
+  <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

[bengr, 2017/02/24 20:58:54]

If you prefer American English, the spelling is canceled. :)

[twifkak, 2017/02/25 01:13:40]

This is one of the cases where I prefer the British spelling (like theatre and
advertisement), but I'm hapy to spel it howevr you wan.

+    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>