[oilpan]: Change marking to do precise roots first and conservative second.

This way we avoid conservatively scanning objects that were reachable through precise pointers.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, sigbjornf@opera.com, tkent@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/405403003

git-svn-id: svn://svn.chromium.org/blink/trunk@178664 bbb929c8-8fbe-4397-9dbb-9b2b20218538