Rename and clarify semantics of GrContext::resolveRenderTarget.

Review URL: https://codereview.chromium.org/696293004
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 075d4d8..da81c69 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -1747,7 +1747,7 @@
 
 void SkGpuDevice::flush() {
     DO_DEFERRED_CLEAR();
-    fContext->resolveRenderTarget(fRenderTarget);
+    fRenderTarget->prepareForExternalRead();
 }
 
 ///////////////////////////////////////////////////////////////////////////////