Offline Pages field trial: use StartsWith for group names
Offline Pages field trial: use StartsWith for group names

This will allow us to have multiple groups within "EnabledAsBookmarks" or "EnabledAsSavedPages" - e.g. "EnabledAsBookmarks-WithNTP" and "EnabledAsBookmarks-WithoutNTP".

BUG=565219

Review URL: https://codereview.chromium.org/1680713002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375157}

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