Rename flushForExternalRead->flushForExternalIO and always call in SkSurface::getTextureHandle

Review URL: https://codereview.chromium.org/1216243003
diff --git a/src/gpu/GrSurface.cpp b/src/gpu/GrSurface.cpp
index b304ccb..9685c56 100644
--- a/src/gpu/GrSurface.cpp
+++ b/src/gpu/GrSurface.cpp
@@ -84,9 +84,9 @@
     }
 }
 
-void GrSurface::prepareForExternalRead() {
+void GrSurface::prepareForExternalIO() {
     if (!this->wasDestroyed()) {
-        this->getContext()->prepareSurfaceForExternalRead(this);
+        this->getContext()->prepareSurfaceForExternalIO(this);
     }
 }