SkRWBuffer: preallocate 'initialCapacity'

src/core/SkRWBuffer.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAtomics.h"
 #include "SkRWBuffer.h"
 #include "SkStream.h"
@@ skipping 163 matching lines @@
             fBlock = nullptr;
         } else {
             fBlock = fBlock->fNext;
         }
     }
     return fRemaining != 0;
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
-SkRWBuffer::SkRWBuffer(size_t initialCapacity) : fHead(nullptr), fTail(nullptr), fTotalUsed(0) {}
+SkRWBuffer::SkRWBuffer(size_t initialCapacity) : fHead(nullptr), fTail(nullptr), fTotalUsed(0) {
+    if (initialCapacity) {
+        fHead = SkBufferHead::Alloc(initialCapacity);
+        fTail = &fHead->fBlock;
+    }
+}
 
 SkRWBuffer::~SkRWBuffer() {
     this->validate();
     if (fHead) {
         fHead->unref();
     }
 }
 
 // It is important that we always completely fill the current block before spilling over to the
 // next, since our reader will be using fCapacity (min'd against its total available) to know how
@@ skipping 151 matching lines @@
     const SkROBuffer*   fBuffer;
     SkROBuffer::Iter    fIter;
     size_t              fLocalOffset;
     size_t              fGlobalOffset;
 };
 
 SkStreamAsset* SkRWBuffer::newStreamSnapshot() const {
     SkAutoTUnref<SkROBuffer> buffer(this->newRBufferSnapshot());
     return new SkROBufferStreamAsset(buffer);
 }