Call SetFocusedTextInputClient in AddedToRootWindow.

We call DetachFromInputMethod in RemovingFromRootWindow when the window is
being removed from the root window because InputMethod object is managed by
the root window.
But RemovingFromRootWindow is also called when the window is moving to another
screen.
So we have to call SetFocusedTextInputClient in AddedToRootWindow which is
called when the window has been moved to another screen.

BUG=325800
TEST=manually done on ChromeOS

Review URL: https://codereview.chromium.org/111443002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239996 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239996