Ifdef guard for SkPdfContext.

Review URL: https://codereview.chromium.org/79723002

git-svn-id: http://skia.googlecode.com/svn/trunk@12332 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/experimental/PdfViewer/inc/SkPdfContext.h b/experimental/PdfViewer/inc/SkPdfContext.h
index 48c5472..7bb9c84 100644
--- a/experimental/PdfViewer/inc/SkPdfContext.h
+++ b/experimental/PdfViewer/inc/SkPdfContext.h
@@ -5,6 +5,9 @@
  * found in the LICENSE file.
  */
 
+#ifndef SkPdfContext_DEFINED
+#define SkPdfContext_DEFINED
+
 #include "SkMatrix.h"
 #include "SkTDStackNester.h"
 #include "SkPdfGraphicsState.h"
@@ -28,3 +31,4 @@
     SkPdfContext(SkPdfNativeDoc* doc);
     ~SkPdfContext();
 };
+#endif // SkPdfContext_DEFINED