Before it depended on util-inl.h. Fix it by moving
MaybeStackBuffer::AllocateSufficientStorage() into
util-inl.h

Drive-by: Replace the SFINAE with a static_assert because we don't
have (or need) an implementation for non-scalar AliasedBufferBase
otherwise. Add forward declarations to memory_tracker.h now that
aliased-buffer.h no longer includes util-inl.h.

Before the AliasedBuffers were represented solely by the TypedArrays
event though they also retain additional memory themselves.
Make the accounting more accurate by implementing MemoryRetainer in
AliasedBuffer.