wm: change XCB_NONE to XCB_MOD_MASK_ANY for button grabs
wm: change XCB_NONE to XCB_MOD_MASK_ANY for button grabs

BUG=chromium-os:4979
TEST=resized panel with modifier keys

Review URL: http://codereview.chromium.org/3046047

joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager