MacViews: "Implement" NativeViewHostMac::GetCursor(). The implementation is a comment.
MacViews: "Implement" NativeViewHostMac::GetCursor(). The implementation is a comment.

The NOTIMPLEMENTED() in NativeViewHostMac::GetCursor(..) generates a lot
of spam. After some investigation, it doesn't need an implementation.

NativeViewHost::GetCursor() appeared in r256934 for Aura, but Mac
(without Aura) doesn't need it. Add a comment explaining the situation.

(See http://crrev.com/1504713009 for a possible implemenatation. It
makes tests happy, but doesn't actually achieve any change in
behaviour.)

BUG=565984

Review URL: https://codereview.chromium.org/1533783002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366036}

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