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Similar to WTF, refactor collections into their own files, allowing
for IWYU but also simplifying the library transition as these data
structures should work with the Blink and the library implementation.

Blink users include `heap_allocator.h` to get the data structures and
not HeapAllocator.

HeapAllocator is now provided through `heap_allocator_impl.h` which is
pulled in by the corresponding data structures.

We cannot work with forward declarations of HeapAllocator, as WTF
doesn't work solely on the type but on Allocator::kIsGarbageCollected.

Bug: 1056170
Change-Id: I1e6f0a481b2e332b0299a6487c41b6b1112f355b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2616561
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841688}