Add UMA metrics for Chrome OS Bluetooth system daemon

This adds the following histograms and their definitions.

[Histograms]
- BlueZ.AdapterLost
- BlueZ.NumberOfExistingAdvertisements

BUG=chromium:724720
TBR=mcchou@chromium.org

(cherry picked from commit b5a0727)

Change-Id: I44e8cb0df131efff57582d5bbbda7abe2b16a97b
Reviewed-on: https://chromium-review.googlesource.com/722076
Commit-Queue: Miao-chen Chou <mcchou@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#509544}
Reviewed-on: https://chromium-review.googlesource.com/728349
Reviewed-by: Miao-chen Chou <mcchou@chromium.org>
Cr-Commit-Position: refs/branch-heads/3239@{#73}
Cr-Branched-From: adb61db-refs/heads/master@{#508578}

Author: mcchou-cr

tools/metrics/histograms/histograms.xml

@@ -7408,6 +7408,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="BlueZ.AdapterLost" units="losses">
+  <owner>mcchou@chromium.org</owner>
+  <summary>
+    This is specific to Chrome OS. Records an occurance of a Bluetooth adapter
+    being lost. This helps us better understand the Bluetooth controller drop
+    issue in the field.
+  </summary>
+</histogram>
+
+<histogram name="BlueZ.NumberOfExistingAdvertisements" units="advertisements">
+  <owner>mcchou@chromium.org</owner>
+  <summary>
+    This is specific to Chrome OS. Records the number of existing advertisements
+    when registering for a new advertisement. This helps us better understand
+    the common amount of advertisments used and the registration failures due to
+    the limited number of advertisement slots.
+  </summary>
+</histogram>
+
 <histogram name="BlueZ.ReasonOfDisconnection" enum="BlueZReasonOfDisconnection">
   <owner>mcchou@chromium.org</owner>
   <summary>