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Main issue: TaskTraits constructor is not robust; any
signature is part of the overload set, even if it may not compile.
Because of that, it may not possible to hold TaskTrait in a tuple (depending on implementation).
Fix: Expose a single constructor that checks if all traits are valid,
or if an extension can accept all traits.
Caveat: An extension has to redefine base TaskTrait::ValidTraits as valid
(through inheritance).
This CL also simplifies querying traits thanks to C++14 relaxed constant expressions.
This reduces compile time complexity of TaskTraits constructor from O(n^3) to O(n^2).


Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I73f993f6be837f1ac665b4b09c78d9d7272435a8
Reviewed-on: https://chromium-review.googlesource.com/c/1194687Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596341}