Doc: update dev/design/pdftheory with new function call

site/dev/design/pdftheory.md

 PDF Theory of Operation
 =======================
 
 <!--
 PRE-GIT DOCUMENT VERSION HISTORY
     2012-06-25 Steve VanDeBogart
                * Original version
     2015-01-14 Hal Canary.
                * Add section "Using the PDF backend"
                * Markdown formatting
@@ skipping 38 matching lines @@
 new SkPDFDevice should be created for each page desired in the
 document. After all the pages have been added to the document,
 `SkPDFDocument::emitPDF()` can be called to get a PDF file. One of the
 special features of the PDF backend is that the same device can be
 added to multiple documents. This for example, would let you generate
 a PDF with the single page you just drew as well as adding it to a
 longer document with a bunch of other pages.
 
 <!--?prettify lang=cc?-->
 
+    SkPDFCanon canon;
     SkAutoUnref<SkPDFDevice> pdfDevice(
-        new SkPDFDevice(width, height, initial_transform));
+        SkPDFDevice::Create(SkISize::Make(width, height), 72.0f, &canon));
 
     SkCanvas canvas(pdfDevice);
     draw_content(&canvas);
 
     SkPDFDocument doc;
     doc.appendPage(dev);
     doc.emitPDF(&pdf_stream);
 
 <a name="PDF_Objects_and_Document_Structure"></a>
 PDF Objects and Document Structure
@@ skipping 517 matching lines @@
 *   [issue 244](http://skbug.com/244)
     Mostly, only TTF fonts are directly supported.  (User metrics 
     show that almost all fonts are truetype.
 *   [issue 260](http://skbug.com/260)
     Page rotation is accomplished by specifying a different
     size page instead of including the appropriate rotation
     annotation.
 
 * * *