Fixing the bounds check in cancelCompositionIfSelectionIsInvalid.

Selection move to the start or the end of the composition node should
not cancel the composition.

BUG=283616

Review URL: https://chromiumcodereview.appspot.com/23848007

git-svn-id: svn://svn.chromium.org/blink/trunk@157345 bbb929c8-8fbe-4397-9dbb-9b2b20218538