Update GC stats, ThreadPool, Timer and ScopedTimer to use monotonic time.

runtime/vm/timer.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_VM_TIMER_H_
 #define RUNTIME_VM_TIMER_H_
 
 #include "platform/utils.h"
 #include "vm/allocation.h"
 #include "vm/atomic.h"
 #include "vm/flags.h"
 #include "vm/os.h"
 
 namespace dart {
 
 // Timer class allows timing of specific operations in the VM.
 class Timer : public ValueObject {
  public:
   Timer(bool report, const char* message) : report_(report), message_(message) {
     Reset();
   }
   ~Timer() {}
 
   // Start timer.
   void Start() {
-    start_ = OS::GetCurrentTimeMicros();
+    start_ = OS::GetCurrentMonotonicMicros();
     running_ = true;
   }
 
   // Stop timer.
   void Stop() {
     ASSERT(start_ != 0);
     ASSERT(running());
-    stop_ = OS::GetCurrentTimeMicros();
+    stop_ = OS::GetCurrentMonotonicMicros();
     int64_t elapsed = ElapsedMicros();
     max_contiguous_ = Utils::Maximum(max_contiguous_, elapsed);
     // Make increment atomic in case it occurs in parallel with aggregation.
     AtomicOperations::IncrementInt64By(&total_, elapsed);
     running_ = false;
   }
 
   // Get total cummulative elapsed time in micros.
   int64_t TotalElapsedTime() const {
     int64_t result = total_;
     if (running_) {
-      int64_t now = OS::GetCurrentTimeMicros();
+      int64_t now = OS::GetCurrentMonotonicMicros();
       result += (now - start_);
     }
     return result;
   }
 
   int64_t MaxContiguous() const {
     int64_t result = max_contiguous_;
     if (running_) {
-      int64_t now = OS::GetCurrentTimeMicros();
+      int64_t now = OS::GetCurrentMonotonicMicros();
       result = Utils::Maximum(result, now - start_);
     }
     return result;
   }
 
   void Reset() {
     start_ = 0;
     stop_ = 0;
     total_ = 0;
     max_contiguous_ = 0;
@@ skipping 99 matching lines @@
   Timer* const timer_;
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(PauseTimerScope);
 };
 
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_TIMER_H_