[CSS Shapes] Simplify RasterShape implementation

Since only floats can specify shape-outside, the RasterShapeIntervals
class only needs to track the first and last above threshold pixel column
(x1 and x2 in the implementation) for each row. Removed code for dealing with
multiple "runs" per row as well as shape-inside internals.

This is a merge of http://trac.webkit.org/changeset/166522 by Hans Muller <hmuller@adobe.com>.

BUG=359098

Review URL: https://codereview.chromium.org/212223006

git-svn-id: svn://svn.chromium.org/blink/trunk@170716 bbb929c8-8fbe-4397-9dbb-9b2b20218538