Unref GrXPFactory.

Not releasing the reference was caught by asan in the Chromium roll.

This fixes "Make GrClipMaskManager stateless and push GrPipelineBuilder construction downstack".

Review-Url: https://codereview.chromium.org/2037193002
diff --git a/src/gpu/GrDrawContext.cpp b/src/gpu/GrDrawContext.cpp
index 581bf08..280e632 100644
--- a/src/gpu/GrDrawContext.cpp
+++ b/src/gpu/GrDrawContext.cpp
@@ -409,7 +409,7 @@
 
     GrPaint paint;
     paint.setAntiAlias(doAA);
-    paint.setXPFactory(GrDisableColorXPFactory::Create());
+    SkSafeUnref(paint.setXPFactory(GrDisableColorXPFactory::Create()));
 
     SkAutoTUnref<GrDrawBatch> batch(fDrawContext->getFillRectBatch(paint, viewMatrix, rect));
     SkASSERT(batch);