Add ViewStructure class temporarily

BUG=497258

Add ViewStructure class from M preview SDK.

Change-Id: Ia743c291f96af1ba5574ddb0aa947cb55c34e467
Reviewed-on: https://chromium-review.googlesource.com/287311
Reviewed-by: Aurimas Liutikas <aurimas@chromium.org>
Commit-Queue: Aurimas Liutikas <aurimas@chromium.org>
Tested-by: Aurimas Liutikas <aurimas@chromium.org>