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

runtime/vm/scope_timer.h

 // Copyright (c) 2015, 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_SCOPE_TIMER_H_
 #define RUNTIME_VM_SCOPE_TIMER_H_
 
 #include "platform/globals.h"
 
 namespace dart {
 
 // Simple utility class for timing a block of code.
 class ScopeTimer : public ValueObject {
  public:
   explicit ScopeTimer(const char* name, bool enabled = true)
       : enabled_(enabled), name_(name), start_(0) {
     if (!enabled_) {
       return;
     }
-    start_ = OS::GetCurrentTimeMicros();
+    start_ = OS::GetCurrentMonotonicMicros();
   }
 
   int64_t GetElapsed() const {
-    int64_t end = OS::GetCurrentTimeMicros();
+    int64_t end = OS::GetCurrentMonotonicMicros();
     ASSERT(end >= start_);
     return end - start_;
   }
 
   ~ScopeTimer() {
     if (!enabled_) {
       return;
     }
     int64_t elapsed = GetElapsed();
     double seconds = MicrosecondsToSeconds(elapsed);
     OS::Print("%s: %f seconds (%" Pd64 " \u00B5s)\n", name_, seconds, elapsed);
   }
 
  private:
   const bool enabled_;
   const char* name_;
   int64_t start_;
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_SCOPE_TIMER_H_