Battery Status API: add UMA logging for Linux.

Adds UMA logging to track how many batteries are reported
by the system at the start of Battery Status API.

BUG=

Review URL: https://codereview.chromium.org/480113003

Cr-Commit-Position: refs/heads/master@{#292398}

content/browser/battery_status/battery_status_manager_linux.cc

@@ -5,6 +5,7 @@
 #include "content/browser/battery_status/battery_status_manager_linux.h"
 
 #include "base/macros.h"
+#include "base/metrics/histogram.h"
 #include "base/threading/thread.h"
 #include "base/values.h"
 #include "content/browser/battery_status/battery_status_manager.h"
@@ -96,6 +97,11 @@ scoped_ptr<PathsVector> GetPowerSourcesPaths(dbus::ObjectProxy* proxy) {
   return paths.Pass();;
 }
 
+void UpdateNumberBatteriesHistogram(int count) {
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "BatteryStatus.NumberBatteriesLinux", count, 1, 5, 6);
+}
+
 // Class that represents a dedicated thread which communicates with DBus to
 // obtain battery information and receives battery change notifications.
 class BatteryStatusNotificationThread : public base::Thread {
@@ -131,6 +137,7 @@ class BatteryStatusNotificationThread : public base::Thread {
         system_bus_->GetObjectProxy(kUPowerServiceName,
                                     dbus::ObjectPath(kUPowerPath));
     scoped_ptr<PathsVector> device_paths = GetPowerSourcesPaths(power_proxy);
+    int num_batteries = 0;
 
     for (size_t i = 0; i < device_paths->size(); ++i) {
       const dbus::ObjectPath& device_path = device_paths->at(i);
@@ -157,14 +164,16 @@ class BatteryStatusNotificationThread : public base::Thread {
         // TODO(timvolodine): add support for multiple batteries. Currently we
         // only collect information from the first battery we encounter
         // (crbug.com/400780).
-        // TODO(timvolodine): add UMA logging for this case.
         LOG(WARNING) << "multiple batteries found, "
                      << "using status data of the first battery only.";
       } else {
         battery_proxy_ = device_proxy;
       }
+      num_batteries++;
     }
 
+    UpdateNumberBatteriesHistogram(num_batteries);
+
     if (!battery_proxy_) {
       callback_.Run(blink::WebBatteryStatus());
       return;

tools/metrics/histograms/histograms.xml

@@ -1927,6 +1927,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="BatteryStatus.NumberBatteriesLinux"
+    enum="BatteryStatusNumberBatteriesLinux">
+  <owner>timvolodine@chromium.org</owner>
+  <summary>
+    Number of batteries reported by the UPower service on Linux at the start of
+    the Battery Status API.
+  </summary>
+</histogram>
+
 <histogram name="BatteryStatus.StartAndroid" enum="BooleanSuccess">
   <owner>timvolodine@chromium.org</owner>
   <summary>
@@ -37848,6 +37857,10 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="2" label="Bad"/>
 </enum>
 
+<enum name="BatteryStatusNumberBatteriesLinux" type="int">
+  <int value="5" label="5+"/>
+</enum>
+
 <enum name="BlacklistSetup" type="int">
   <int value="0" label="Blacklist enabled"/>
   <int value="1" label="Blacklist ran successfully."/>