Add run-time CHECK to smoke-test allocator overrides
As part of the refactoring work on the allocator, it emerged that it
would be nice to have a test that ensures that we don't accidentally
break things along the way and not detected that.
This CL adds a runtime CHECK() that verifies that:
- On Windows (non-component build) the shim layer has been
initialized. Concretely this checks that chrome called the shimmed
_heap_init() method and not the one from libcmt.
- On Linux desktop, the malloc symbols are being overridden by tcmalloc
(only when USE_TCMALLOC is defined).
The rationale of this test is: if _heap_init was shimmed there are
very good chances that malloc (& friends) were shimmed as well.
Likewise on Linux for mallopt() <-> malloc().