[DebugVisualizer] Fix recent changes in NGPhysicalFragment

This patch fixes DebugVisualizer for recent changes in
NGPhysicalFragment.

Because NGPhysicalFragment has no virtual functions, VS
cannot resolve correct subclasses. This change includes
resolving to the correct subclasses, and fix to visualize
children.

Verified by enabling DebugVisualizer warnings in VS.

Change-Id: I31aea119235667c9a40f614c042f8efbe16466b7
Reviewed-on: https://chromium-review.googlesource.com/c/1418530
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623824}
1 file changed