Replace WebDocument::focusedNode to  focusedElement

WebDocument::focusedNode is a deprecated Api, move chromium to new api focusedElement.

BUG=258763

Review URL: https://codereview.chromium.org/165233003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253703 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253703