Bindings: Separate WorldIdConstants to WorldTypes and WorldId
This is a clean-up CL and doesn't change behavior.
For improving extensibility, this CL separates WorldIdConstants to 2 parts:
WorldTypes and WorldId. This encapsulates identifier allocation logic in
DOMWrapperWorld[1] and makes it easier to expand the identifier space for
Worklets[2].
[1] WorldIds for IsolatedWorlds still need to be given from out of
DOMWrapperWorld because of its unique convention to allocate the identifier.
[2] https://codereview.chromium.org/2735823006/
BUG=697622, 697629
Review-Url: https://codereview.chromium.org/2735973006
Cr-Commit-Position: refs/heads/master@{#455700}
8 files changed
