CRASH calling chromium malloc_usable_size() when jemalloc not selected

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 02, 2011 13:51:53_

running chromium's unit_tests.exe, if anything but jemalloc is selected, malloc_usable_size() will crash (b/c it's part of jemalloc).
the problem is that heap routine selection is done inside of malloc(), so to Dr. Memory there is a malloc() and malloc_usable_size() pair, yet as it turns out they do not go together since malloc() calls into tcmalloc, and the size routine to pair with it in that case is tc_malloc_usable_size().
yet malloc_usable_size() does not perform dynamic heap selection and is hardocded for jemalloc.

xref issue #267

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=314_