Fix a style guide violation in KeyedService.
Fix a style guide violation in KeyedService.

According to the style guide [1], constructors and destructor should come
before any other method declaration. Fixes KeyedService method ordering
to follow said guide.

[1]: https://google.github.io/styleguide/cppguide.html#Declaration_Order

BUG=None

Review URL: https://codereview.chromium.org/1459843002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#360797}

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