Oilpan: Actually relax the emptiness assertion for conservative stack scanning.

My previous patch was exceptionally incorrect: it only relaxed the
assertion if there was a direct pointer from the stack to an object
but not if something was traced indirectly from a pointer found on
the stack.

This patch disables the assert for all heap pages if any object
is found during conservative scanning at all. This is a shame
because we have the risk that this is basically disabling the
assertion completely. We should consider if we can keep better
track of this in debug mode so we can get this assertion in
play more.

R=erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, zerny@chromium.org
BUG=375671

Review URL: https://codereview.chromium.org/299193002

git-svn-id: svn://svn.chromium.org/blink/trunk@174801 bbb929c8-8fbe-4397-9dbb-9b2b20218538