Make timeline use the same clock source as mojo's tracing infrastructure

runtime/vm/os.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 VM_OS_H_
 #define VM_OS_H_
 
 #include "vm/globals.h"
 
 // Forward declarations.
@@ skipping 29 matching lines @@
   static int GetLocalTimeZoneAdjustmentInSeconds();
 
   // Returns the current time in milliseconds measured
   // from midnight January 1, 1970 UTC.
   static int64_t GetCurrentTimeMillis();
 
   // Returns the current time in microseconds measured
   // from midnight January 1, 1970 UTC.
   static int64_t GetCurrentTimeMicros();
 
+  // Returns the current time used by the tracing infrastructure.
+  static int64_t GetCurrentTraceMicros();
+
   // Returns a cleared aligned array of type T with n entries.
   // Alignment must be >= 16 and a power of two.
   template<typename T>
   static T* AllocateAlignedArray(intptr_t n, intptr_t alignment) {
     T* result = reinterpret_cast<T*>(OS::AlignedAllocate(n * sizeof(*result),
                                                          alignment));
     memset(result, 0, n * sizeof(*result));
     return result;
   }
 
@@ skipping 86 matching lines @@
   static void Shutdown();
 
   static void Abort();
 
   static void Exit(int code);
 };
 
 }  // namespace dart
 
 #endif  // VM_OS_H_