Update TestExpectation for svg/custom/getsvgdocument.html

After r167574, svg/custom/getsvgdocument.html stops failing and becomes image-only-failure.

BUG=284734

TBR=kouhei

Review URL: https://codereview.chromium.org/175163002

git-svn-id: svn://svn.chromium.org/blink/trunk@167597 bbb929c8-8fbe-4397-9dbb-9b2b20218538