Dump layout selection test result as text.

This patch implements |std::string DumpSelectionInfo()| so that
we can check a SelectionStatus tree with text diff.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Ib42cebfc5a7f1908a4e748f622bbdae9c24987c6
Reviewed-on: https://chromium-review.googlesource.com/1136335
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575948}
1 file changed