Oilpan: Scale up the number of marking threads according to available processors

Currently we use 2 marking threads regardless of the number of processors available.
Given that the marking phase takes a substantial amount of time,
this CL changes the number of marking threads to min(the number of processors, 4).

This CL also decreases the block size of each CallStack from 200 to 128
because load balancing is not good in some benchmarks such as LargeDistributionWithoutLayout. This CL improves LargeDistributionWithoutLayout by 10% for example.

BUG=420515

Review URL: https://codereview.chromium.org/648633002

git-svn-id: svn://svn.chromium.org/blink/trunk@183613 bbb929c8-8fbe-4397-9dbb-9b2b20218538