GitLab

Style guide requires explicit constructors.

Prior to this patch, a T* can be passed into a function that requires
a scoped_nsobject<T>, and the object will be released but not retained.

Also make the copy constructor that takes a different typename more
generic, allowing for different types (e.g, subclasses) in addition to
different traits.

Clean up all instances where we relied on this behavior.

BUG=

Review-Url: https://codereview.chromium.org/2028823003
Cr-Commit-Position: refs/heads/master@{#397847}