Tidy up the allocator shim, which is now Windows only.

      Only re-implement the functions that we really need to re-implement to perform our additional checks.  Let the CRT do the rest.  Mark specifically which functions from which CRT files we are re-implementing.  Add missing ones like _query_new_mode.

Fold generic_allocators.cc into allocator_shim_win.cc as this code is now very Windows specific and no point being generic any more.

This is a dependent CL to let ffmpegsumo start using the allocator shims, since it needs _aligned_realloc which the allocator shim never implemented.  Instead, let the CRT do this for us, since it just calls our malloc().

BUG=434397,450676,446032,444533
TEST=base_unittests --gtest_filter=SecurityTest.*
TEST=allocator_unittests

Review URL: https://codereview.chromium.org/868533003

Cr-Commit-Position: refs/heads/master@{#312720}