[FragmentItem] Move GetChildOffset to NGBoxFragmentBuilder

This patch moves |GetChildOffset| from
|NGContainerFragmentBuilder| to |NGBoxFragmentBuilder|. The
function needs to read |NGFragmentItemsBuilder| to support
|NGFragmentItems| but the member is only available in
|NGBoxFragmentBuilder|.

All existing usages are for |NGBoxFragmentBuilder|, so no
changes to callers.

This patch has no behavior changes.

Bug: 982194
Change-Id: Icacd18d12634bc4324d5f56eb70710903e7d942a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076745
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744943}
4 files changed