Make previousLinePosition() not to use dangling RootInlineBox

This patch makes |previousLinePosition()| not to use dangling |RootInlineBox|
pointer to avoid use-after-free.

Before this patch, |isEditablePosition()| is called with |DoUpdateStyle|
parameter to update layout tree if needed. Usually, layout tree isn't updated
by this |isEditablePosition()| call since |previousLinePosition()| updates
layout tree at entry. However, if there are pending style sheet, e.g. @import
directive, and HTML import, e.g link rel=import, layout tree is updated since
document isn't rendering ready, |haveImportLoaded()| &&


Cr-Commit-Position: refs/heads/master@{#401231}
2 files changed