Expose RenderFrameHost::GetLastCommittedOrigin().

It is an error to call this unless the RFH is current -- similar to
GetLastCommittedURL, but we actually enforce it.

There are two pending CLs that will want to use this: 1413853005 and 1465533008.

BUG=498580
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/1475433002

Cr-Commit-Position: refs/heads/master@{#361722}

• content/browser/frame_host/frame_tree_browsertest.cc[diff]
• content/browser/frame_host/render_frame_host_impl.cc[diff]
• content/browser/frame_host/render_frame_host_impl.h[diff]
• content/public/browser/render_frame_host.h[diff]
• content/test/data/frame_tree/top.html[diff]
• url/origin.h[diff]
• url/scheme_host_port.h[diff]