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To avoid using the BrowserThread we instantiate a ThreadChecker instance on the NetLogObserver::Attach method and use this to validate that calls to the NetLogObserver instance occur on the same thread.

To avoid using ContentBrowserClient, we pass the NetLog pointer in the NetLogObserver::Attach method.

BUG=598073

Review-Url: https://codereview.chromium.org/2115823004
Cr-Commit-Position: refs/heads/master@{#405284}