mojo filesystem: Simplify full file reading/writing.

components/filesystem/directory_impl.h

Index: components/filesystem/directory_impl.h
diff --git a/components/filesystem/directory_impl.h b/components/filesystem/directory_impl.h
index 1e4c903443ea3ef564d687639fd99802c50adb2e..4f5b1a4ddd5e0a9f12460d34325d9077a134f5ed 100644
--- a/components/filesystem/directory_impl.h
+++ b/components/filesystem/directory_impl.h
@@ -55,6 +55,11 @@ class DirectoryImpl : public Directory {
   void IsWritable(const mojo::String& path,
                   const IsWritableCallback& callback) override;
   void Flush(const FlushCallback& callback) override;
+  void ReadEntireFile(const mojo::String& path,
+                      const ReadEntireFileCallback& callback) override;
+  void WriteFile(const mojo::String& path,
+                 mojo::Array<uint8_t> data,
+                 const WriteFileCallback& callback) override;
 
  private:
   mojo::StrongBinding<Directory> binding_;