Make base::PowerMonitor work on Chrome OS.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index d4283ae4eb876237b27b5bcc63ce6104d974f4d5..35b9e13c77d0a5dec60b53c28a5c2e0dcd564c6c 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -445,6 +445,7 @@ component("base") {
     "power_monitor/power_monitor_device_source.h",
     "power_monitor/power_monitor_device_source_android.cc",
     "power_monitor/power_monitor_device_source_android.h",
+    "power_monitor/power_monitor_device_source_chromeos.cc",
     "power_monitor/power_monitor_device_source_ios.mm",
     "power_monitor/power_monitor_device_source_mac.mm",
     "power_monitor/power_monitor_device_source_posix.cc",
@@ -802,6 +803,12 @@ component("base") {
     ]
   }
 
+  if (is_chromeos) {
+    sources -= [
+      "power_monitor/power_monitor_device_source_posix.cc",
+    ]
+  }
+
   if (is_nacl) {
     # These things would otherwise be built on a Posix build but aren't
     # supported on NaCl.