Stacked floats with shape-outside should allow inline content to interact with the non-outermost float

Make inline content interact with stacked floats with shape-outside per
the spec. This means that content can interact with floats on the line
that are not the outermost float.

This refactors ComputeFloatOffsetAdapter into a superclass and two
subclasses: one adaptor for determining the offset for float layout, and
one for determining the offset for inline layout.

The logic in LineWidth::shrinkAvailableWidthForNewFloatIfNeeded has been
updated to handle stacked floats with shape-outside properly and has
been considerably simplified in the process. It was previously doing a
whole bunch of unnecessary work.

This is a port from WebKit of http://trac.webkit.org/changeset/162217

BUG=249140

Review URL: https://codereview.chromium.org/149513005

git-svn-id: svn://svn.chromium.org/blink/trunk@168155 bbb929c8-8fbe-4397-9dbb-9b2b20218538