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This CL adds a BASE_EXPORT to EnabledStateObserver class. This CL is a
part of the effort to get NetLog data into tracing. Since
EnabledStateObserver is not base exported, we would like it to be so
that we can use it outside base.

BUG=399701

Review URL: https://codereview.chromium.org/551333003

Cr-Commit-Position: refs/heads/master@{#293943}