Bound RasterShapeInterval size to int

Everything in the code assumes that the interval size will always fit in
an int. However, to make sure there are no negatives, the
RasterShapeInterval constructor takes an unsigned. This clamps that
value to make sure we can't overflow an int later.

Review URL: https://codereview.chromium.org/313393004

git-svn-id: svn://svn.chromium.org/blink/trunk@175823 bbb929c8-8fbe-4397-9dbb-9b2b20218538