Fix use-after-free in BreakingContext.

BreakingContext was holding a raw pointer in m_currentStyle, but it calls layout during handleFloat(), and FastTextAutosizer assigns a new RenderStyle during layout.  (We can't mutate the existing RenderStyle due to style sharing.)

To ensure that raw pointers to the RenderStyle are safe during layout, FastTextAutosizer now keeps a RefPtr to the previous style until the end of layout.

BUG=360163

Review URL: https://codereview.chromium.org/226283010

git-svn-id: svn://svn.chromium.org/blink/trunk@171013 bbb929c8-8fbe-4397-9dbb-9b2b20218538