Optimize PODFreeListArena::freeObject by using a single shared free list between chunks, rather than each chunk containing it's own free list. This makes ::freeObject O(1) rather than O(n) for the number of allocated chunks. Brings average page load time of oilevent.com (from tough_layout_cases) from ~20s to ~1.7s on my z620.

Also moved some tests from Source/web to Source/core.

BUG=260903
TEST=Run tough_layout_cases page cycler, should load reasonably quickly now:

tools/perf/run_measurement --browser=release page_cycler tools/perf/page_sets/tough_layout_cases.json --page-filter=oil

Review URL: https://chromiumcodereview.appspot.com/19815003

git-svn-id: svn://svn.chromium.org/blink/trunk@155447 bbb929c8-8fbe-4397-9dbb-9b2b20218538