Use ChildThreadImpl::thread_safe_sender in font proxy if available

content/child/dwrite_font_proxy/dwrite_font_proxy_win.h

Index: content/child/dwrite_font_proxy/dwrite_font_proxy_win.h
diff --git a/content/child/dwrite_font_proxy/dwrite_font_proxy_win.h b/content/child/dwrite_font_proxy/dwrite_font_proxy_win.h
index d6115cdab6e430c31ae085e68d61c906cfd22030..0f2ddd19ea0ac303e10261212315214b0bc1ef3e 100644
--- a/content/child/dwrite_font_proxy/dwrite_font_proxy_win.h
+++ b/content/child/dwrite_font_proxy/dwrite_font_proxy_win.h
@@ -64,7 +64,8 @@ class CONTENT_EXPORT DWriteFontCollectionProxy
                       IDWriteFontFileStream** font_file_stream) override;
 
   HRESULT STDMETHODCALLTYPE
-  RuntimeClassInitialize(IDWriteFactory* factory, IPC::Sender* sender_override);
+  RuntimeClassInitialize(IDWriteFactory* factory,
+                         const base::Callback<IPC::Sender*(void)>& sender);
 
   void Unregister();
 
@@ -82,13 +83,11 @@ class CONTENT_EXPORT DWriteFontCollectionProxy
   bool CreateFamily(UINT32 family_index);
 
  private:
-  IPC::Sender* GetSender();
-
   Microsoft::WRL::ComPtr<IDWriteFactory> factory_;
   std::vector<Microsoft::WRL::ComPtr<DWriteFontFamilyProxy>> families_;
   std::map<base::string16, UINT32> family_names_;
   UINT32 family_count_ = UINT_MAX;
-  IPC::Sender* sender_override_;
+  base::Callback<IPC::Sender*(void)> sender_;
 
   DISALLOW_ASSIGN(DWriteFontCollectionProxy);
 };