Fix negative shadow repaint issue

When a box with negative v-shadow changes height, the repaint rect
is inadequate resulting some part of shadow is not repainted.
Fix the issue by considering negative shadow-top/shadow-left.

TEST=fast/box-shadow/negative-shadow-box-expand.html
TEST=fast/box-shadow/negative-shadow-box-shrink.html

BUG=330141

Review URL: https://codereview.chromium.org/110893011

git-svn-id: svn://svn.chromium.org/blink/trunk@164709 bbb929c8-8fbe-4397-9dbb-9b2b20218538