Dump LayoutObject directly for LocalCaretRect unit tests

Historically, we can't dump a LayoutObject to ostream, so LocalCaretRect
unit tests dump the Node of a LayoutObject instead. Now that we don't
have this restriction, this patch changes the tests to dump LayoutObject
directly to be more explicit, and also allow dumping null LayoutObjects.

Change-Id: I9a63aed186cdea3caf172a44a6001a4a797fc63b
Reviewed-on: https://chromium-review.googlesource.com/963709Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543626}

third_party/WebKit/Source/core/editing/LocalCaretRectTest.cpp

@@ -19,7 +19,7 @@ bool operator==(const LocalCaretRect& rect1, const LocalCaretRect& rect2) {
 }
 
 std::ostream& operator<<(std::ostream& out, const LocalCaretRect& caret_rect) {
-  return out << "layout_object = " << caret_rect.layout_object->GetNode()
+  return out << "layout_object = " << caret_rect.layout_object
              << ", rect = " << caret_rect.rect;
 }