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This adds a function to PPB_View which allows plugins to know the scroll offset
of the page when they are in view. This is useful for OOP PDF which uses the
scroll offset of the window it is contained in to determine the document's
scroll location. A web page can send scroll location via postMessage but
this is slow. Sending the offset directly via view messages is much faster
and seems reasonable.

We don't send the scroll offset in the cases where the plugin is off screen
to avoid any more additional IPC traffic than what currently exists.

BUG=303491

Review URL: https://codereview.chromium.org/329033003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278236 0039d316-1c4b-4281-b951-d872f2087c98