Rework startup-data-util.

Rework startup-data-util.

- Make the API look like v8::V8::InitializeICU.
  (That is: A static method call, not an object to be created on the stack.)
- Fix path separator on Windows, by calling base::OS::isPathSeparator.
- Move into API, so that it can be called by hello-world & friends.
- Actually call it from hello-world and friends.

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1292053002

Cr-Commit-Position: refs/heads/master@{#30174}