Enable author shadows for SVG elements

This patch enables author shadows for SVG elements. The referenced bug
no longer applies (SVG's tref was removed) and the shadow code has matured
since. Tests have been added for <g>, <rect>, and <svg> elements.

BUG=416217

Review URL: https://codereview.chromium.org/585183004

git-svn-id: svn://svn.chromium.org/blink/trunk@183441 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: fix build after r183426.

TBR=haraken,oilpan-reviews
BUG=383125
NOTRY=true

Review URL: https://codereview.chromium.org/639133002

git-svn-id: svn://svn.chromium.org/blink/trunk@183442 bbb929c8-8fbe-4397-9dbb-9b2b20218538