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In UIWebView world CookieStoreIOS could dynamically change its state
and either be synchronized with NSHTTPCookieStorage or unsynchronized.

In WKWebView world CookieStoreIOS never changes its state. It is either
always synchronized (for sign in) or always unsynchronized (for regular
browsing).

Notable changes:
- updated comments to reflect that sync state is not dynamic
- removed RoundTripTest which tested non-existing dynamic synchronization flow
- removed other tests which tests dynamic synchronization

BUG=676144

Review-Url: https://codereview.chromium.org/2604763003
Cr-Commit-Position: refs/heads/master@{#440851}