Make SharedMemory track the size that was actually mapped

ui/surface/transport_dib_linux.cc

 // 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.
 
 #include "ui/surface/transport_dib.h"
 
 #include <errno.h>
 #include <stdlib.h>
 #include <sys/ipc.h>
 #include <sys/shm.h>
@@ skipping 74 matching lines @@
 bool TransportDIB::is_valid_handle(Handle dib) {
   return dib >= 0;
 }
 
 // static
 bool TransportDIB::is_valid_id(Id id) {
   return id.shmkey != -1;
 }
 
 skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
-  if (address_ == kInvalidAddress && !Map())
+  if ((address_ == kInvalidAddress && !Map()) || !VerifyCanvasSize(w, h))
     return NULL;
   return skia::CreatePlatformCanvas(w, h, true,
                                     reinterpret_cast<uint8_t*>(memory()),
                                     skia::RETURN_NULL_ON_FAILURE);
 }
 
 bool TransportDIB::Map() {
   if (!is_valid_id(key_))
     return false;
   if (address_ != kInvalidAddress)
@@ skipping 40 matching lines @@
   if (!inflight_counter_ && detached_)
     delete this;
 }
 
 void TransportDIB::Detach() {
   CHECK(!detached_);
   detached_ = true;
   if (!inflight_counter_)
     delete this;
 }