Add non-inline SkRTreeFactory constructor to fix clang-cl

src/core/SkBBHFactory.cpp

Index: src/core/SkBBHFactory.cpp
diff --git a/src/core/SkBBHFactory.cpp b/src/core/SkBBHFactory.cpp
index e8534cacb33c8c9d048147a49a4c228599d7f879..b10dc985f8208ccd80c9a41b89cd14311a38e3d8 100644
--- a/src/core/SkBBHFactory.cpp
+++ b/src/core/SkBBHFactory.cpp
@@ -14,3 +14,7 @@ SkBBoxHierarchy* SkRTreeFactory::operator()(const SkRect& bounds) const {
     SkScalar aspectRatio = bounds.width() / bounds.height();
     return new SkRTree(aspectRatio);
 }
+
+// TODO(thakis@chromium): remove once HTTP://llvm.org/26506 is fixed
+SkRTreeFactory::SkRTreeFactory() {
+}