Record UMA for battery level when precache starts and ends

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 f306e4efd6977bf52abe6738a3ef10dc6c9ab778..5258f2b33b8ad9a2668063fc64dd2403e20fd345 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -44774,6 +44774,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Precache.BatteryPercentage.Start" units="%">
+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    The battery level in percentage when the precache task was started.
+  </summary>
+</histogram>
+
+<histogram name="Precache.BatteryPercentageDiff.End" units="%">

[sclittle, 2016/09/14 16:44:44]

I thought precaching only happens when the device is charging?

If it's being changed to happen when the device isn't charging, then should we
record something special here for when power is connected vs. when it isn't?

E.g., if you're experimenting with doing precaching even when the device isn't
charging, then you'll probably want to demonstrate that when the device isn't
charging, precaching doesn't use up too much power. Wouldn't you need to have a
histogram for measuring the battery percentage difference when the device isn't
charging, instead of having one that combines measurements for when it's
charging and not charging?

It's probably good enough to check if the device is charging when precaching
starts (or when precaching ends, if that's easier). There's probably no need to
do something more complicated to handle the case where the device is
connected/disconnected from power while it's precaching.

Or are you planning to do something special when analyzing this histogram to
answer this question without making this distinction?

[Raj, 2016/09/14 16:50:57]

Yes. Currently it happens only when device is charging.
The plan is to disable power requirement only in DEV and find the power usage.
I will disable the power checks in a separate CL.
After analyzing the power usage, we could further disable power checks in
STABLE.

[sclittle, 2016/09/14 17:17:05]

Acknowledged.

+  <owner>bengr@chromium.org</owner>
+  <owner>rajendrant@chromium.org</owner>
+  <summary>
+    Measures the difference between the battery level percentage when the
+    precache task is started and ended. This value indicates the possible
+    battery usage due to the precache task. Logged when the precache task ends,
+    which could be due to successful completion, time-out, error conditions, max
+    download limit exceeded, etc.
+  </summary>
+</histogram>
+
 <histogram name="Precache.CacheStatus.NonPrefetch" enum="HttpCachePattern">
   <owner>jamartin@chromium.org</owner>
   <owner>bengr@chromium.org</owner>