Reland r166634: Do not scan the active part of the stack in oilpan conservative garbage collections.

With a fix to ensure that stack iteration always starts from a properly aligned address otherwise we attempt to read past stack boundary.

Original description:

Do not scan the active part of the stack in oilpan conservative garbage collections.

If threads are allowed to continue after safepointing there is a
stack marker. We should stop stack scanning at the marker in that
case. If there is no marker the thread whose thread we are iterating
will be blocked and we should scan all the way to the recorded
end stack pointer.

Review URL: https://codereview.chromium.org/158893003

git-svn-id: svn://svn.chromium.org/blink/trunk@166838 bbb929c8-8fbe-4397-9dbb-9b2b20218538