Make SkTDArray accessors const-friendly

src/pdf/SkPDFDocument.cpp

Index: src/pdf/SkPDFDocument.cpp
===================================================================
--- src/pdf/SkPDFDocument.cpp	(revision 7881)
+++ src/pdf/SkPDFDocument.cpp	(working copy)
@@ -43,7 +43,7 @@
         usage.merge(pages[i]->getFontGlyphUsage());
     }
     SkPDFGlyphSetMap::F2BIter iterator(usage);
-    SkPDFGlyphSetMap::FontGlyphSetPair* entry = iterator.next();
+    const SkPDFGlyphSetMap::FontGlyphSetPair* entry = iterator.next();
     while (entry) {
         SkPDFFont* subsetFont =
             entry->fFont->getFontSubset(entry->fGlyphSet);