Add ReloadPageLoadMetricsObserver to have a metrics variant for reloads

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 15bd59778cc5f97fb8cc636c7e6d730001186c66..f7952e4799139337d870675ac6dd0e5e8997a631 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -36155,6 +36155,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Reload.NavigationToFirstContentfulPaint" units="ms">

[Bryan McQuade, 2016/05/25 22:53:29]

if we switch to PageLoad.Reload.PaintTiming.NavigationToFirstContentfulPaint or
PageLoad.PaintTiming.NavigationToFirstContentfulPaint.Reload, you can use a
histogram suffix instead.

[Takashi Toyoshima, 2016/05/26 06:06:50]

This is my first time to use suffix rules in the histograms.xml, but hum....
format isn't clear enough for me to use it for my cases.

Here are some questions:
- PageLoadMetricsClients histogram_suffixes tag contains mixed lines of
affected-histogram tags and suffix tags in random order. How each instance
affects each other?
- I could not find a document for with-suffix tag, but how can I use it to have
"foo.bar" and "foo.bar.suffix".
- Header document says we can use histogram_suffixes recursively with max depth
limit 5. But, how can I use it recursively? If I can not use with-suffix for the
previous case, I'll use it recursively to add a suffix.
- Is there any way to dump preprocessed xml that expand suffix related tags?
- I can not find most suffixed metric names in the UMA dashboard. Why?

+  <owner>ksakamoto@chromium.org</owner>
+  <owner>toyoshim@chromium.org</owner>
+  <summary>
+    The time from navigation start to first &quot;contentful&quot; paint only on

[Ilya Sherman, 2016/05/25 23:13:01]

nit: "paint only" -> "paint. Only recorded"

+    pages that are initiated by one of reload operations.

[Ilya Sherman, 2016/05/25 23:13:01]

nit: "by one of reload operations" -> "by a reload operation" (or just "reload",
or "reload action", I'm not sure what the best term for that is).

+  </summary>
+</histogram>
+
 <histogram name="PageLoad.Timing.NavigationToDOMContentLoadedEventFired"
     units="ms">
   <owner>bmcquade@chromium.org</owner>