Change FetchGCS function type to exported.
Change FetchGCS function type to exported.

The FetchGCS function type was recently added to add a handler for
fetching files from GCS. The type was not exported, making it impossible
for functions to return a handler of the correct type. This makes the
type exported.

R=maruel@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2715223004

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