Count beforeload separately for isolated worlds

BUG=308218

Review URL: https://codereview.chromium.org/26828003

git-svn-id: svn://svn.chromium.org/blink/trunk@159909 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[CSS Shapes] Improve the performance of image valued shapes with large shape-margins

The typical cost of computing the shape-margin boundary of an image-valued shape-outside is now proportional to (2 * shape-margin + image.height) rather than (2 * shape-margin * image.height). The performance improvement comes from skipping sequences of rounded-rectangle intervals that will not contribute to the final result. Each non-empty row in the original image contributes one rounded-rectangle whose corner radius is shape-margin, height is 2 * shape-margin, and width is 2 * shape-margin plus the width of the limits of the intervals on the row.

Renamed private method RasterShape::getIntervals() to intervalsAt() per feedback from an earlier review.

BUG=306270

Review URL: https://codereview.chromium.org/26887009

git-svn-id: svn://svn.chromium.org/blink/trunk@159910 bbb929c8-8fbe-4397-9dbb-9b2b20218538