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Since |SSLErrorHandler| was a |WebContentsUserData| tied to the lifetime of a
|WebContents|, it wasn't properly deleted when the |WebContents| was reloaded
or navigated to another page. This CL removes the error handler explicitly and
adds browser tests to assert reload/navigate behavior.

BUG=453875

Review URL: https://codereview.chromium.org/1004283004

Cr-Commit-Position: refs/heads/master@{#321603}