Oilpan: introduce sticky forcedForTesting flag to ensure that a precise
GC is performed at the next return to the event loop.

Use this in svg animation tests to be able to reliably check that
objects die when expected.

R=erik.corry@gmail.com, haraken@chromium.org, kouhei@chromium.org, oilpan-reviews@chromium.org, vegorov@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/220203005

git-svn-id: svn://svn.chromium.org/blink/trunk@170648 bbb929c8-8fbe-4397-9dbb-9b2b20218538