[FastTextAutosizer] Create a temporary cluster stack for list items

The fast text autosizer maintains a stack of clusters that's normally
kept up-to-date in beginLayout (including synthesizing previous
clusters during a subtree layout). This patch fixes an edge-case we
missed: inflateListItem can also be called without a cluster stack.
This patch creates a temporary cluster stack in inflateListItem if
needed. A similar case does not exist in inflateTable because table
inflation goes through beginLayout.

This patch also contains a minor rename for clarity:
m_firstBlock -> m_firstBlockToBeginLayout

BUG=360171

Review URL: https://codereview.chromium.org/227273002

git-svn-id: svn://svn.chromium.org/blink/trunk@171308 bbb929c8-8fbe-4397-9dbb-9b2b20218538