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This patch enables LayoutNGPaintFragments in virtual/layout_ng,
and also merges layout_ng and layout_ng_paint.

There are additional failures; some are just to be rebaselined.
I will analyze and cleanup in following patches.

Bug: 792329, 714962
Change-Id: Ie61d9672e3d4f44847ef8771214c16e993245943
Reviewed-on: https://chromium-review.googlesource.com/804803Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522469}