Move prepareForExternalIO implementation from GrContext to GrDrawingManager

src/gpu/GrContext.cpp

Index: src/gpu/GrContext.cpp
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 357f58e6e323240234a8f226fd0e65b3ea2a76c3..47fb32312f42684ca389083851ec53c96f71349b 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -518,13 +518,7 @@ void GrContext::prepareSurfaceForExternalIO(GrSurface* surface) {
     RETURN_IF_ABANDONED
     SkASSERT(surface);
     ASSERT_OWNED_RESOURCE(surface);
-    if (surface->surfacePriv().hasPendingIO()) {
-        this->flush();
-    }
-    GrRenderTarget* rt = surface->asRenderTarget();
-    if (fGpu && rt) {
-        fGpu->resolveRenderTarget(rt);
-    }
+    fDrawingManager->prepareSurfaceForExternalIO(surface);
 }
 
 bool GrContext::copySurface(GrSurface* dst, GrSurface* src, const SkIRect& srcRect,