Add comments explaining Task Manager idle wakeups computation.

base/process/process_metrics_mac.cc

Index: base/process/process_metrics_mac.cc
diff --git a/base/process/process_metrics_mac.cc b/base/process/process_metrics_mac.cc
index 7fc931748ae3fa292c45fd509a9fd81db24a0a27..a3c2d6a14aa6acbd2e07e9afd6a55a836bbb3f10 100644
--- a/base/process/process_metrics_mac.cc
+++ b/base/process/process_metrics_mac.cc
@@ -324,6 +324,17 @@ int ProcessMetrics::GetIdleWakeupsPerSecond() {
     // where TASK_POWER_INFO isn't supported yet.
     return 0;
   }
+
+  // The task_power_info struct contains two wakeup counters:
+  // task_interrupt_wakeups and task_platform_idle_wakeups.
+  // task_interrupt_wakeups is the total number of wakeups generated by the
+  // process, and is the number that Activity Monitor reports.
+  // task_platform_idle_wakeups is a subset of task_interrupt_wakeups that
+  // tallies the number of times the processor was taken out of its low-power
+  // idle state to handle a wakeup. task_platform_idle_wakeups therefore result
+  // in a greater power increase than the other interrupts which occur while the
+  // CPU is already working, and reducing them has a greater overall impact on
+  // power usage. See the powermetrics man page for more info.
   return CalculateIdleWakeupsPerSecond(
       power_info_data.task_platform_idle_wakeups);
 }