Add explicit keyword to RawPtr(T&).
Add explicit keyword to RawPtr(T&).

Also, fix a comment.

BUG=

Review URL: https://codereview.chromium.org/189413003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@168716 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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