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

Review URL: https://codereview.chromium.org/1216243003
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index ed588b6..5cb8934 100755
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -593,7 +593,7 @@
     return true;
 }
 
-void GrContext::prepareSurfaceForExternalRead(GrSurface* surface) {
+void GrContext::prepareSurfaceForExternalIO(GrSurface* surface) {
     RETURN_IF_ABANDONED
     SkASSERT(surface);
     ASSERT_OWNED_RESOURCE(surface);