[layoutng] Implement CloneWithoutOffsets for fragments and layout results

This is a prerequisite for fragment caching (https://crrev.com/c/516042/)

Bug: 635619
Change-Id: I8d4995082b638c7073513dc2b11f4d01934fbe4a
Reviewed-on: https://chromium-review.googlesource.com/575652
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487524}
8 files changed