[Clean up] Fix the comment for SiteIntance::CreateForURL
[Clean up] Fix the comment for SiteIntance::CreateForURL

In SiteIntance::CreateForURL there will certainly create a new SiteInstance
and BrowsingIntance since in this new BrowsingIntance we will not find any
existing SiteIntance. So BrowsingIntance will never be deleted even beyond
the scope of the method.
The original comments will make some confusion.

BUG=11629

Review URL: https://codereview.chromium.org/880303004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#315308}

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