Remove CheckFocusedElementTask

Previously, we'd spin posting the CheckFocusedElementTask waiting for style
recalc and layout to occur.  After this CL, we check whether the focused element
is no longer focusable at the end of recalc style.  We still fire the blur events
asynchronously, however.

BUG=340455

Review URL: https://codereview.chromium.org/155433003

git-svn-id: svn://svn.chromium.org/blink/trunk@166551 bbb929c8-8fbe-4397-9dbb-9b2b20218538