Hello WebKit folks!

I would like to announce that I’ve just landed the patch which introduces 
`WTF::makeUnique<T>` and `WTF::makeUniqueWithoutFastMallocCheck<T>` in 
https://trac.webkit.org/changeset/248846 
<https://trac.webkit.org/changeset/248846>.
They are drop-in-replacement to std::make_unique<T>, and we should not use 
std::make_unique<T> after that patch is introduced.
I’m planning to add cpplint check for `std::make_unique` to avoid the use of 
that.

The motivation behind this change is the following.

1. Our typical way of allocating heap memory is three-fold. Using containers 
(Vector etc.), RefCounted, and std::unique_ptr.
2. Containers and RefCounted are covered well by FastMalloc.
3. But std::unique_ptr case, we missed using FastMalloc in many places so far.

Even in very recently written code, we missed FastMalloc annotation. For 
example, we sometimes create a data structure just like a struct, and allocate 
it with make_unique.

struct XXXData {
    ...
};

m_data = std::make_unique<XXXData>();

We missed WTF_MAKE_STRUCT_FAST_ALLOCATED annotation in XXXData so frequently so 
that the allocation of XXXData ends up being allocated from system-malloc.

This WTF::makeUnique adds one `static_assert` over std::make_unique: the 
static_assert ensures T is FastMalloced or IsoHeap-allocated.
Otherwise, we see compile-error. This mechanism surprisingly found so many 
classes that do not have WTF_MAKE_FAST_ALLOCATED / 
WTF_MAKE_STRUCT_FAST_ALLOCATED in our code base.

If the type T comes from ThirdParty and if we cannot annotate T with 
FAST_ALLOCATED, we can use WTF::makeUniqueWithoutFastMallocCheck explicitly as 
a fallback.

More detailed explanation behind why we took this design (instead of allocating 
FastMalloced-memory automatically when using makeUnique<T>() etc.) is described 
in ChangeLog in https://trac.webkit.org/changeset/248846/webkit 
<https://trac.webkit.org/changeset/248846/webkit>.
I already annotated missed structs / classes with WTF_MAKE_FAST_ALLOCATED in 
https://trac.webkit.org/changeset/248762 
<https://trac.webkit.org/changeset/248762>. So, now I think 99% of allocations 
in WebKit-itself are handled well by FastMalloc.

-Yusuke
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