Increase Web Bluetooth device scanning duration

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 4e1dd217e61e72b9e2e5c2caa24e46af81b4cdd4..321d244cd640daf716a0af1fec61f9042b5e0e80 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -4819,6 +4819,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Bluetooth.Web.RequestDevice.ScanningDuration" units="ms">
+  <owner>jyasskin@chromium.org</owner>
+  <owner>ortuno@chromium.org</owner>
+  <owner>scheib@chromium.org</owner>
+  <owner>juncai@chromium.org</owner>
+  <summary>
+    Records how long it takes for finding at least one Bluetooth device.

[ortuno, 2016/11/18 00:06:52]

I think the description might be wrong?

[scheib, 2016/11/18 00:07:30]

Records the duration scanning for devices is run, terminated by events such as
the chooser being closed with a selected device, canceled, or the scan duration
timing out.

[juncai, 2016/11/18 00:30:01]

Done.

[juncai, 2016/11/18 00:30:02]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Bluetooth.Web.RequestDevice.UnionOfServices.Count"
     units="services">
   <owner>jyasskin@chromium.org</owner>
@@ -104060,6 +104070,7 @@ value.
   <int value="16" label="Web Bluetooth kill switch enabled"/>
   <int value="17" label="Web Bluetooth chooser event handler invalid"/>
   <int value="18" label="Bluetooth Low Energy not available"/>
+  <int value="19" label="Chooser re-scan link pressed."/>

[scheib, 2016/11/18 00:07:30]

This is the wrong location to add this entry. Add to
Bluetooth.Web.RequestDevice.Outcome. Also update the description there
clarifying that multiple outcomes may result for a given RequestDevice, such as
Rescan multiple times and then Select.

[juncai, 2016/11/18 00:30:01]

In the code review diff it seems that it is under:
"Bluetooth.Web.RequestDevice.UnionOfServices.Count", but it is not. It is under
the Bluetooth.Web.RequestDevice.Outcome:
enum="WebBluetoothRequestDeviceOutcome".

I updated the description for the Bluetooth.Web.RequestDevice.Outcome.

 </enum>
 
 <enum name="WebBluetoothRSSISignalStrengthLevel" type="int">