Move inlined methods to .h files accordingly, follwing style guide
rule:
https://google.github.io/styleguide/cppguide.html#Self_contained_Headers

Bug: chromium:1056170
Change-Id: Ia6c4f82bd4352d507eece36e540ad0d318e56920
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2273858Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68627}