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Miscellaneous changes:
- IWYU fixes for the library;
- Move some needed traits to global includes;
- Add stub for invoking a testing GC;

Bug: 1056170
Change-Id: Ia4871852a4f73595d0c9466140d7fb7cd7bd6e61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624729
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842706}