[CSS Shapes] Improve the performance of image valued shapes

Replaced the implementation of RasterShapeIntervals with one based on the new ShapeInterval<int> class. This eliminates the dependency the Region class and delivers a 10X layout speedup for large image valued shapes.

BUG=278343

Review URL: https://chromiumcodereview.appspot.com/23904013

git-svn-id: svn://svn.chromium.org/blink/trunk@157976 bbb929c8-8fbe-4397-9dbb-9b2b20218538