Makes RootWindow::RepostEvent() target capture window

Prior to this RepostEvent would always look up the target based on
position, circumventing capture. It shouldn't do that.

BUG=275760
TEST=covered by unit test
R=sadrul@chromium.org

Review URL: https://codereview.chromium.org/26762002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227940 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227940