Add PrintedPage::GetCenteredPageContentRect so Mac and Win can share code.

printing/printed_page.cc

Index: printing/printed_page.cc
diff --git a/printing/printed_page.cc b/printing/printed_page.cc
index 8abedeee846660fca3d6724cfa9bc478d4d724ba..1661813b78996529958c756890bcb968018c0b76 100644
--- a/printing/printed_page.cc
+++ b/printing/printed_page.cc
@@ -25,4 +25,17 @@ const NativeMetafile* PrintedPage::native_metafile() const {
   return native_metafile_.get();
 }
 
+void PrintedPage::GetCenteredPageContentRect(
+    const gfx::Size& paper_size, gfx::Rect* content_rect) const {
+  *content_rect = page_content_rect();
+  if (paper_size.width() > page_size().width()) {
+    int diff = paper_size.width() - page_size().width();
+    content_rect->set_x(content_rect->x() + diff / 2);
+  }
+  if (paper_size.height() > page_size().height()) {
+    int diff = paper_size.height() - page_size().height();
+    content_rect->set_y(content_rect->y() + diff / 2);
+  }
+}
+
 }  // namespace printing