Be consistent when applying outline style to anonymous block continuation

Anonymous block continuation should use its continuation's style for
outline.

However, previously we only use continuation's style during
invalidation, but not when painting outline.

Rename RenderObject::outlineStyleForPaintInvalidation() to
RenderObject::outlineStyle(), and use it when applicable.

BUG=388799
TEST=fast/css/anonymous-block-continuation-outline.html

Review URL: https://codereview.chromium.org/363083003

git-svn-id: svn://svn.chromium.org/blink/trunk@177599 bbb929c8-8fbe-4397-9dbb-9b2b20218538