Remove #pragma once. Just from base/ for now.

base/time.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Time represents an absolute point in time, internally represented as
 // microseconds (s/1,000,000) since a platform-dependent epoch.  Each
 // platform's epoch, along with other system-dependent clock interface
 // routines, is defined in time_PLATFORM.cc.
 //
 // TimeDelta represents a duration of time, internally represented in
 // microseconds.
 //
 // TimeTicks represents an abstract time that is always incrementing for use
 // in measuring time durations. It is internally represented in microseconds.
 // It can not be converted to a human-readable time, but is guaranteed not to
 // decrease (if the user changes the computer clock, Time::Now() may actually
 // decrease or jump).
 //
 // These classes are represented as only a 64-bit value, so they can be
 // efficiently passed by value.
 
 #ifndef BASE_TIME_H_
 #define BASE_TIME_H_
-#pragma once
 
 #include <time.h>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/basictypes.h"
 
 #if defined(OS_POSIX)
 // For struct timeval.
 #include <sys/time.h>
@@ skipping 554 matching lines @@
 #endif
 };
 
 inline TimeTicks TimeDelta::operator+(TimeTicks t) const {
   return TimeTicks(t.ticks_ + delta_);
 }
 
 }  // namespace base
 
 #endif  // BASE_TIME_H_