Move prepareForExternalIO from GrRenderTarget to GrDrawContext

src/gpu/GrSurface.cpp

Index: src/gpu/GrSurface.cpp
diff --git a/src/gpu/GrSurface.cpp b/src/gpu/GrSurface.cpp
index 6dc90a3d1aea07b0353601b9a5eb6f2977f66096..824a6a2936e9ebf886c0f39aace7c293d5033428 100644
--- a/src/gpu/GrSurface.cpp
+++ b/src/gpu/GrSurface.cpp
@@ -149,12 +149,6 @@ void GrSurface::flushWrites() {
     }
 }
 
-void GrSurface::prepareForExternalIO() {
-    if (!this->wasDestroyed()) {
-        this->getContext()->prepareSurfaceForExternalIO(this);
-    }
-}
-
 bool GrSurface::hasPendingRead() const {
     const GrTexture* thisTex = this->asTexture();
     if (thisTex && thisTex->internalHasPendingRead()) {