CL 2697123007 added a context argument to most of the shim allocator
functions. This means that it is no longer valid to define (say) malloc as
an alias of ShimMalloc, because their function types no longer match.

This change defines the allocator functions as real functions that call the
shims passing nullptr as the context.

Found with -fsanitize=cfi-icall.

R=primiano@chromium.org
BUG=696986,601497

Review-Url: https://codereview.chromium.org/2720703004
Cr-Commit-Position: refs/heads/master@{#453900}