setCompositionFromExistingText() causes two calls to AutofillClient.

Due to the way setCompositionFromExistingText was implemented it was triggering
two textChange updates due to a call to execCommand("delete"). This call is not
needed as setComposition replaces the selection anyway.

BUG=263453

Review URL: https://chromiumcodereview.appspot.com/21052005

git-svn-id: svn://svn.chromium.org/blink/trunk@155216 bbb929c8-8fbe-4397-9dbb-9b2b20218538