Temporarily disable parallel marking

This CL disables parallel marking because the current implementation is slower than a single-threaded marking in most benchmarks. We confirmed it in both Linux and Mac. For example, this CL improves parser/querySelector-* by 30%, shadow-dom/MultipleInsertionPoints by 20% etc.

The reason of the slowness is that the current implementation of parallel marking is not cache-aware. Marking threads write mark bits on objects owned by the main thread and thus pollute CPU caches.

We are planning to implement bitmap marking to address the cache problem, but it must be implemented in a way that the new implementation is faster than a single-threaded version. Thus, it would make sense to simply disable parallel marking at this point and try to implement advanced marking algorithm in a follow-up CL.

BUG=420515

Review URL: https://codereview.chromium.org/666673011

git-svn-id: svn://svn.chromium.org/blink/trunk@184146 bbb929c8-8fbe-4397-9dbb-9b2b20218538