Add SK_API to GrBackendTexture and GrBackendRenderTarget

Change-Id: Ibae248bf8e2d2da860e99b62346e737da55a85b5
Reviewed-on: https://skia-review.googlesource.com/21827
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/include/gpu/GrBackendSurface.h b/include/gpu/GrBackendSurface.h
index 60a9eb5..cf5d82b 100644
--- a/include/gpu/GrBackendSurface.h
+++ b/include/gpu/GrBackendSurface.h
@@ -15,7 +15,7 @@
 #include "vk/GrVkTypes.h"
 #endif
 
-class GrBackendTexture {
+class SK_API GrBackendTexture {
 public:
     GrBackendTexture(int width,
                      int height,
@@ -64,7 +64,7 @@
     };
 };
 
-class GrBackendRenderTarget {
+class SK_API GrBackendRenderTarget {
 public:
     GrBackendRenderTarget(int width,
                           int height,