Fix improper usage of the sequence checker in WMIRefresher

WMIRefresher is created during the creation of SystemMonitor on the UI
thread, it is then used only in a MayBlock sequence.

Change-Id: I7a14f59272a7ac6170c25e15518f31bfbc9f4870
Reviewed-on: https://chromium-review.googlesource.com/c/1493157
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636884}

chrome/browser/performance_monitor/wmi_refresher.cc

@@ -72,7 +72,9 @@ enum class WMIRefresher::RefreshStatus {
 };
 
 WMIRefresher::WMIRefresher() : initialized_called_(false) {
-  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+  // This object might be created on a sequence different than the one on which
+  // it'll be used.
+  DETACH_FROM_SEQUENCE(sequence_checker_);
 }
 
 WMIRefresher::~WMIRefresher() = default;

chrome/browser/performance_monitor/wmi_refresher.h

@@ -22,8 +22,11 @@ namespace win {
 // Wrapper around an IWbemRefresher instance used to retrieve some system
 // performance counters.
 //
-// This object should live on a MayBlock() sequence. Concurrent calls to
-// RefreshAndGetDiskIdleTimeInPercent are not allowed.
+// The constructor and InitializeDiskIdleTimeConfig() don't have any sequencing
+// requirements. Calls to RefreshAndGetDiskIdleTimeInPercent() must happen on
+// the same sequence, after InitializeDiskIdleTimeConfig() has returned. All
+// methods except the constructor must be invoked in a context that accepts
+// blocking calls (e.g. a MayBlock() sequence).
 //
 // This class relies on the fact that by default all sequences are in a
 // multithreaded COM apartment.
@@ -33,8 +36,7 @@ class WMIRefresher {
   ~WMIRefresher();
 
   // Initialize everything needed to retrieve the disk idle time data via WMI.
-  // This should only be called once. This can be called on a sequence different
-  // than the one on which this object has been created.
+  // This should only be called once.
   //
   // Returns true on success, false otherwise.
   bool InitializeDiskIdleTimeConfig();