add SK_API to forward declare in canvas.h

NOTRY=True
TBR=

Author: reed@google.com

Review URL: https://codereview.chromium.org/541073002
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 3043a3e..6b2266a 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -40,7 +40,7 @@
 class SkCanvasState;
 
 namespace SkCanvasStateUtils {
-    SkCanvasState* CaptureCanvasState(SkCanvas*);// needs gettotalclip()
+    SK_API SkCanvasState* CaptureCanvasState(SkCanvas*);
 }
 
 /** \class SkCanvas
@@ -1310,7 +1310,8 @@
     friend class SkDeferredDevice;  // needs getTopDevice()
     friend class SkSurface_Raster;  // needs getDevice()
     
-    friend SkCanvasState* SkCanvasStateUtils::CaptureCanvasState(SkCanvas*);// needs gettotalclip()
+    // needs gettotalclip()
+    friend SkCanvasState* SkCanvasStateUtils::CaptureCanvasState(SkCanvas*);
     
     SkBaseDevice* createLayerDevice(const SkImageInfo&);