Made setWholeSubtreeInvalid non-inline.

Fix for review issue in https://codereview.chromium.org/177653003/ raised
after it landed.

R=ojan
BUG=335247

Review URL: https://codereview.chromium.org/179483002

git-svn-id: svn://svn.chromium.org/blink/trunk@167827 bbb929c8-8fbe-4397-9dbb-9b2b20218538