Make reallocation of large objects reliable.

If ThreadHeap::reallocate<T>() is called with a size that's
equal or greater to the large object threshold (64k, currently),
make sure it ends up being allocated as a large object.

Large objects would previously be attempted allocated on a normal arena,
hoping that the allocation couldn't be serviced by bump allocation, but
fall into an out-of-line allocation.

R=
BUG=

Review-Url: https://codereview.chromium.org/2019273002
Cr-Commit-Position: refs/heads/master@{#396712}
3 files changed