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

gin/isolate_holder.cc

Index: gin/isolate_holder.cc
diff --git a/gin/isolate_holder.cc b/gin/isolate_holder.cc
index 317b5822bd8f47f773a0927d1a2b18ef6c2ebcb8..85672a2c7f64d1c6d700c423f42aae89c5716ed4 100644
--- a/gin/isolate_holder.cc
+++ b/gin/isolate_holder.cc
@@ -194,6 +194,28 @@ bool IsolateHolder::LoadV8SnapshotFd(int natives_fd,
   return MapV8Files(
       NULL, NULL, natives_fd, snapshot_fd, natives_region, snapshot_region);
 }
+
+// static
+bool IsolateHolder::OpenV8FilesForChildProcesses(
+    base::PlatformFile* natives_fd_out,
+    base::PlatformFile* snapshot_fd_out) {
+  base::FilePath data_path;
+  PathService::Get(kV8SnapshotBasePathKey, &data_path);
+  DCHECK(!data_path.empty());
+
+  int file_flags = base::File::FLAG_OPEN | base::File::FLAG_READ;
+  base::FilePath natives_data_path = data_path.AppendASCII(kNativesFileName);
+  base::FilePath snapshot_data_path = data_path.AppendASCII(kSnapshotFileName);
+  base::File natives_data_file(natives_data_path, file_flags);
+  base::File snapshot_data_file(snapshot_data_path, file_flags);
+
+  if (!natives_data_file.IsValid() || !snapshot_data_file.IsValid())
+    return false;
+
+  *natives_fd_out = natives_data_file.TakePlatformFile();
+  *snapshot_fd_out = snapshot_data_file.TakePlatformFile();
+  return true;
+}
 #endif  // V8_USE_EXTERNAL_STARTUP_DATA
 
 //static