Oilpan: Simplify Heap::allocate

This CL simplifies Heap::allocate for an upcoming optimization (I'm planning to implement size-aware Heap). This CL doesn't improve performance.

- The key change is in Heap::allocate.

- After this CL, ThreadState::getStats and ThreadState::getStatsForTesting return different stats. ThreadState::getStats returns the size of allocated objects *including* object headers, whereas ThreadState::getStatsForTesting returns the size of allocated objects *excluding* object headers. This is because there is no easy way to know the header size of each object in tests, and thus tests cannot test the size of allocated objects unless ThreadState::getStatsForTesting returns the size of allocated objects *excluding* object headers.

- This CL renames Heap::getStats to Heap::getStatsForTesting because it is used by testing only.

BUG=420515

Review URL: https://codereview.chromium.org/634243004

git-svn-id: svn://svn.chromium.org/blink/trunk@184285 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Paint Profiler: fix selection size & hieght in paint log window

BUG=426095

Review URL: https://codereview.chromium.org/645403004

git-svn-id: svn://svn.chromium.org/blink/trunk@184286 bbb929c8-8fbe-4397-9dbb-9b2b20218538