Lacros: Preparation for lacros target_os switch.

This is a mechanical change replacing defined(OS_CHROMEOS) with
BUILDFLAG(IS_CHROMEOS_ASH) to ensure switching lacros' target_os
from linux to chromeos does not affect the lacros binary.

More detail on http://go/lacros-macros.

Bug: 1052397
Change-Id: I96c231186a89d259a6fd339ebf72a144ff98b6af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2628634Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844060}

base/power_monitor/power_monitor_device_source.h

@@ -130,7 +130,7 @@ class BASE_EXPORT PowerMonitorDeviceSource : public PowerMonitorSource {
   PowerMessageWindow power_message_window_;
 #endif
 
-#if defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_CHROMEOS_ASH)
   PowerObserver::DeviceThermalState current_thermal_state_ =
       PowerObserver::DeviceThermalState::kUnknown;
 #endif