Fix off by one in creating a RasterShape

marginY1 is supposed to be an inclusive endpoint, however, maxY() is an
exclusive endpoint. To fix this, subtract one from maxY() before
assiging to marginY1 so that we don't walk off the end of the intervals
vector.

BUG=379108
R=betravis@adobe.com,pdr@chromium.org,inferno@chromium.org

Review URL: https://codereview.chromium.org/318443004

git-svn-id: svn://svn.chromium.org/blink/trunk@175433 bbb929c8-8fbe-4397-9dbb-9b2b20218538