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The text autosizer demarcates certain nodes as "clusters" that autosize
using a consistent font multiplier (see http://tinyurl.com/TextAutosizer
for a full description). Clusters can nest forming a tree, and a root
cluster is assumed to be available to all Nodes except the LayoutView.

This patch fixes a bug where no root cluster was created by a LayoutView
which contained LayoutObject children (in this case, a sole pagination
anonymous block) but no Node children. A similar situation can occur
with completely empty documents.

Asserts & comments have been added to clarify the root cluster concept.

BUG=521657

Review URL: https://codereview.chromium.org/1308693003

git-svn-id: svn://svn.chromium.org/blink/trunk@201172 bbb929c8-8fbe-4397-9dbb-9b2b20218538