Remove unnecessary memcpy() in XMLDocumentParser.cpp.

This also removes SharedBuffer::moveTo() given that XMLDocumentParser.cpp is
the only client.

SharedBuffer::moveTo() was initially introduced to save a memcpy() in
ResourceFetcher. Unfortunately the code has been moved around and also changed
quite significantly leading to the addition of an unnecessary copy constructor
call defeating the purpose of the optimization in the first place.

BUG=290193

Review URL: https://codereview.chromium.org/152813002

git-svn-id: svn://svn.chromium.org/blink/trunk@167613 bbb929c8-8fbe-4397-9dbb-9b2b20218538