Add UMA for High Resolution Timer Usage

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 d66fda78add604022d3e48a52b2c8b2c786867ad..54646d2fdba1ff7aff05a52fd1da8b3acc3c9bc7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -86616,6 +86616,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Windows.HighResolutionTimerUsage" units="%">
+  <owner>stanisc@chromium.org</owner>
+  <summary>
+    Percentage of elapsed time the high resolution timer is activated for each

[rkaplow, 2017/06/27 21:49:18]

Pretty minor - I think this can be rephrased in a way that is a bit easier to
grok. So this is record on 10m intervals - so this is the percentage of that
interval that it is activated? And not sure how this works for 'each' Chrome
process - does each record seperately?

[stanisc, 2017/06/27 22:07:31]

Yeah, the high resolution timer is actually activated if any process requests
that, but I don't think we can join the histogram data from all process easily
without doing something complicated like sending all the data over IPC to the
browser process and joining all the activation intervals there. So we thought
that it isn't worth the complexity and it should be good enough to just log the
UMA from each process individually and let the the data be aggregated at the
histogram level. 

How about this version:
Percentage of elapsed time the high resolution timer is activated. The usage is
reported by each of Chrome processes individually (without aggregation) and
logged every 10 minutes.

[stanisc, 2017/06/28 22:28:35]

Done.

+    Chrome process. Logged every 10 minutes.
+  </summary>
+</histogram>
+
 <histogram name="Windows.InCompatibilityMode" enum="BooleanCompatibilityMode">
   <owner>scottmg@chromium.org</owner>
   <summary>