Add support to extension_shell and ash_shell to use external V8 snapshot files.

gin/public/isolate_holder.h

 // Copyright 2013 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.
 
 #ifndef GIN_PUBLIC_ISOLATE_HOLDER_H_
 #define GIN_PUBLIC_ISOLATE_HOLDER_H_
 
 #include "base/basictypes.h"
+#include "base/files/file.h"
 #include "base/memory/scoped_ptr.h"
 #include "gin/gin_export.h"
 #include "v8/include/v8.h"
 
 namespace gin {
 
 class PerIsolateData;
 class RunMicrotasksObserver;
 
 // To embed Gin, first initialize gin using IsolateHolder::Initialize and then
@@ skipping 30 matching lines @@
 
   // This method should also only be called once, and on the MessageLoop's
   // thread.
   void RemoveRunMicrotasksObserver();
 
 #if defined(V8_USE_EXTERNAL_STARTUP_DATA)
   static const int kV8SnapshotBasePathKey;
   static const char kNativesFileName[];
   static const char kSnapshotFileName[];
 
+  // Load the V8 snapshot data from the given file descriptors. If offset and
+  // size are zero it will load the whole file, otherwise it will map and load
+  // the region of the file specified by offset and size. Returns true on
+  // success.

[James Cook, 2015/03/19 15:48:37]

Thanks for documenting the other methods!

   static bool LoadV8SnapshotFd(int natives_fd,
                                int64 natives_offset,
                                int64 natives_size,
                                int snapshot_fd,
                                int64 snapshot_offset,
                                int64 snapshot_size);
+
+  // Load the V8 snapshot data from the snapshot files. Returns true on success.
   static bool LoadV8Snapshot();
+
+  // Opens the V8 snapshot data files and returns open file descriptors to these
+  // files in |natives_fd_out| and |snapshot_fd_out|, which can be passed to
+  // child processes.
+  static bool OpenV8FilesForChildProcesses(base::PlatformFile* natives_fd_out,
+                                           base::PlatformFile* snapshot_fd_out);
+
 #endif  // V8_USE_EXTERNAL_STARTUP_DATA
   static void GetV8ExternalSnapshotData(const char** natives_data_out,
                                         int* natives_size_out,
                                         const char** snapshot_data_out,
                                         int* snapshot_size_out);
 
  private:
   v8::Isolate* isolate_;
   scoped_ptr<PerIsolateData> isolate_data_;
   scoped_ptr<RunMicrotasksObserver> task_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(IsolateHolder);
 };
 
 }  // namespace gin
 
 #endif  // GIN_PUBLIC_ISOLATE_HOLDER_H_