Improve FEDisplacement performance.

When traversing the DAG for clipping opportunities in
determineAbsolutePaintRect(), don't union FEDisplacement's result with
the size its displacement input ("in2"). This is irrelevant in
computing the minimum paint rect, since the resulting image can only
be the size of the color input ("in") inflated by m_scale, which
FEDisplacementMap::mapPaintRect() already does.

This gives a 43X speedup on IE's PirateMark (673.59s -> 15.41s).

BUG=367163

Review URL: https://codereview.chromium.org/515903002

git-svn-id: svn://svn.chromium.org/blink/trunk@181053 bbb929c8-8fbe-4397-9dbb-9b2b20218538