Reland "Add support for range-based for loops to SkTHashSet/Map."

Updated Pair type in SkTHashMap to derive from std::pair to fix C++14
issues with structured bindings.

Original change's description:
> Add support for range-based for loops to SkTHashSet/Map.
>
> This allows loops over SkTHashes to break in the middle, and also
> removes the need to use lambda captures to bring variables inside the
> loop's scope.
>
> Change-Id: Ief55d776b2c57a44b24cfe1c94493a5d514791c8
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/346496
> Reviewed-by: Mike Klein <mtklein@google.com>
> Commit-Queue: John Stiles <johnstiles@google.com>

Change-Id: I2ac5b2c59e70ed0ec3b42b32e7994d6bcdf56b40
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/346502
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
4 files changed