Handle all allocations internally to partitionAllocGeneric().

We now no longer fall back to fastMalloc() for larger allocation sizes. The
benefits include:

- The hacky SuperPageBitmap optimization can be completely removed.
- We remove a couple of hot / tricky to predict branches in the free path.
- The realloc() logic becomes a lot simpler.
- For very large allocations, we fully guard page them.
- Some minor simplifications to the management of extent lists.

- And we now make progress on removing the Blink copy of tcmalloc for Mac.
- Also, we can make progress on graceful failure of large ArrayBuffers.

BUG=327095
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/105503009

git-svn-id: svn://svn.chromium.org/blink/trunk@164447 bbb929c8-8fbe-4397-9dbb-9b2b20218538