Add Precache.Fetch.Begin user action.

tools/metrics/actions/actions.xml

Index: tools/metrics/actions/actions.xml
diff --git a/tools/metrics/actions/actions.xml b/tools/metrics/actions/actions.xml
index bdae8ad0c5a539bf471c2395d593dd54ad929074..bb586edf67a5cdb7df359d9f6d9fadb2a2a6e38f 100644
--- a/tools/metrics/actions/actions.xml
+++ b/tools/metrics/actions/actions.xml
@@ -13834,6 +13834,14 @@ should be able to be added at any place in this file.
   <obsolete>Deprecated due to NPAPI removal.</obsolete>
 </action>
 
+<action name="Precache.Fetch.Begin" not_user_triggered="true">
+  <owner>twifkak@chromium.org</owner>
+  <owner>jamartin@chromium.org</owner>
+  <description>
+    A precache fetch was initiated, having met all the necessary preconditions.

[Alexei Svitkine (slow), 2017/04/10 19:04:10]

Please expand description to explain why this is needed as an action and not as
a histogram.

In general, for not user triggered things, histograms are preferred - unless you
want to do some kind of custom analysis. Can you detail this in the description?

[twifkak, 2017/04/11 01:15:43]

I added a bit of detail to the CL description. Let me know if you'd like more
detail on how the analysis is being done -- I can provide that offline.

I didn't add it to the description in the XML, because it seemed out of place
compared to other descriptions in this file. (And out of place for the UMA
frontend.) I'll add it if you feel otherwise, though.

+  </description>
+</action>
+
 <action name="Precache.Fetch.IntentReceived" not_user_triggered="true">
   <owner>twifkak@chromium.org</owner>
   <owner>bengr@chromium.org</owner>