Refactor CanvasRenderingContext2D::scrollPathIntoViewInternal().

This fixes a crash when a RenderBoxModelObject is null.
Also, we can simplify the logic to take border and padding into account by using absoluteContextBox().

BUG=391185
TEST=https://cluster-fuzz.appspot.com/testcase?key=6636208132194304

Review URL: https://codereview.chromium.org/367113007

git-svn-id: svn://svn.chromium.org/blink/trunk@177625 bbb929c8-8fbe-4397-9dbb-9b2b20218538