Correct line width calculation with inline-block induced word spacing

A previous CL allowed word spacing to be added around inline-blocks.
However the line width calculation wasn't consistent with children placement,
and contents will overflow on the right side of the line if there was
inline-block induced word spacing.

This CL makes RenderBlockFlow::computeInlineDirectionPositionsForSegment to be
consistent with InlineFlowBox::placeBoxRangeInInlineDirection.

BUG=429655

Review URL: https://codereview.chromium.org/704693003

git-svn-id: svn://svn.chromium.org/blink/trunk@184853 bbb929c8-8fbe-4397-9dbb-9b2b20218538