Refactor RenderBox::addVisualEffectOverflow()

Extracted computeVisualEffectOverflow(). It will be used in later
changes about invalidation of visual effect overflow.

BUG=380472
TEST=none (refactor only)

Review URL: https://codereview.chromium.org/337473008

git-svn-id: svn://svn.chromium.org/blink/trunk@177368 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Don't force full invalidation on padding or margin change

The renderer can already correctly handle invalidations of geometry changes 
caused by change of padding and/or margin. 

BUG=375876
TEST=fast/repaint/padding-margin.html

Review URL: https://codereview.chromium.org/340323002

git-svn-id: svn://svn.chromium.org/blink/trunk@177369 bbb929c8-8fbe-4397-9dbb-9b2b20218538