Remove obviously unneeded forward declarations from headers under src/{app,base,net}.

base/histogram.h

 // Copyright (c) 2006-2010 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.
 
 // Histogram is an object that aggregates statistics, and can summarize them in
 // various forms, including ASCII graphical, HTML, and numerically (as a
 // vector of numbers corresponding to each of the aggregating buckets).
 
 // It supports calls to accumulate either time intervals (which are processed
 // as integral number of milliseconds), or arbitrary integral units.
@@ skipping 204 matching lines @@
   } while (0)
 
 //------------------------------------------------------------------------------
 
 class BooleanHistogram;
 class CustomHistogram;
 class Histogram;
 class LinearHistogram;
 class Pickle;
 
-namespace disk_cache {
-  class StatsHistogram;
-};  // namespace disk_cache
-
-
 class Histogram : public base::RefCountedThreadSafe<Histogram> {
  public:
   typedef int Sample;  // Used for samples (and ranges of samples).
   typedef int Count;  // Used to count samples in a bucket.
   static const Sample kSampleType_MAX = INT_MAX;
 
   typedef std::vector<Count> Counts;
   typedef std::vector<Sample> Ranges;
 
   /* These enums are meant to facilitate deserialization of renderer histograms
@@ skipping 381 matching lines @@
   // lock protects access to the above map.
   static Lock* lock_;
 
   // Dump all known histograms to log.
   static bool dump_on_exit_;
 
   DISALLOW_COPY_AND_ASSIGN(StatisticsRecorder);
 };
 
 #endif  // BASE_HISTOGRAM_H_