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This is the internal promise representation. It implements the machinery
needed to marshal and dispatch promises as they become ready for
execution.

Care has been taken to try and minimize the size of the AbstractPromise
class and to keep down the number of heap allocations. The promise result
and the executor are both stored in the base::unique_any since they are
never needed at the same time. The size of AbstractPromise on x64 is 96
bytes in builds without DCHECKS.

In builds with DCHECKS it's larger because there's additional storage
used to diagnose and prevent various usage hazards:

* Unhandled rejection
* Double move of promise results to callbacks
* Mixed move and non-move semantics of promise results to callbacks

The base::any_internal type has been adjusted so it can store the
SmallUniqueObject<> inline, which is used to store the promise executor.
The largest anticipated promise executor base::All is the size of
3x sizeof(void*) to hold a vtable and a std::vector.

Design: https://docs.google.com/document/d/1l12PAJgEtlrqTXKiw6mk2cR2jP7FAfCCDr-DGIdiC9w/edit

TBR=fdoray@chromium.org, etiennep@chromium.org

Bug: 906125
Change-Id: I62cecf23ecc8ea3ef7417d6ad40c0911fac5339c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607643Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658982}