[LayoutNG] Remove unintuitive side effects in NGInlineItemsBuilder::ToString

NGInlineItemsBuilder::ToString() removes trailing spaces at
the end of the block. This is unintuitive from the function
name, and caused CL:1322184 to be reverted.

This patch moves this to ExitBlock(), and make ToString()
clean, without side effects.

Ran webkit_unit_tests locally with DCHECK disabled.

Bug: 636993
Change-Id: I03f814afc272b9be0c274391cde076aecb6dbc91
Reviewed-on: https://chromium-review.googlesource.com/c/1329804
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607579}
3 files changed