Call setChildrenInline(true) only for blocks with inline children.

Currently, all blocks have this bit set to true, and then non-block-flows
turn around and set it back to false.

BUG=393945

Review URL: https://codereview.chromium.org/393823002

git-svn-id: svn://svn.chromium.org/blink/trunk@178328 bbb929c8-8fbe-4397-9dbb-9b2b20218538