Remove FragementationData lazyness

Having fragmentation data be lazy on RootInlineBox only saves us a single
int (due to the pointer and the fact that we have room to pack its bool
into InlineFlowBox's bitfields), and the allocation overhead was large enough
it appeared on performance tests (see bug).

BUG=313700

Review URL: https://codereview.chromium.org/715973004

git-svn-id: svn://svn.chromium.org/blink/trunk@185170 bbb929c8-8fbe-4397-9dbb-9b2b20218538