[CSS Shapes] Add support for shape-outside image values

Added minimal support for shape-outside image values. A new method that computes the excluded intervals for a horizontal line segment was added to the RasterIntervals class. The stub RasterShape getExcludedIntervals() method has been replaced by one that uses the new method.

Image shapes are represented by a RasterIntervals object, which just encapsulates a Region object. The new getExcludedIntervals() method computes the excluded intervals for a horizontal line segment between y1 and y2. To find the excluded intervals we vertically expand each of the image shape Region's rectangles that fall within the line  segment, so that they begin at y1 and have height = y2 - y1. The union of the expanded rectangles produces a new Region whose horizontal projection defines the excluded intervals.

BUG=273226

Review URL: https://chromiumcodereview.appspot.com/23117004

git-svn-id: svn://svn.chromium.org/blink/trunk@156136 bbb929c8-8fbe-4397-9dbb-9b2b20218538