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

gin/public/isolate_holder.h

Index: gin/public/isolate_holder.h
diff --git a/gin/public/isolate_holder.h b/gin/public/isolate_holder.h
index 7968b1df0ba3edd8a9978105b927a494ec4cb5ef..fb9aea8b5c5afb91df5e85cb3a3b4bc409d01a89 100644
--- a/gin/public/isolate_holder.h
+++ b/gin/public/isolate_holder.h
@@ -10,6 +10,16 @@
 #include "gin/gin_export.h"
 #include "v8/include/v8.h"
 
+namespace base {
+
+// Forward declaration of PlatformFile from base/file/file.h
+#if defined(OS_WIN)
+typedef HANDLE PlatformFile;
+#elif defined(OS_POSIX)
+typedef int PlatformFile;
+#endif
+}
+
 namespace gin {
 
 class PerIsolateData;
@@ -52,17 +62,26 @@ class GIN_EXPORT IsolateHolder {
   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.
   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,