Verify that appendChild(script) does not perform a microtask checkpoint

This potential bug was raised in the review for the previous patch:
https://codereview.chromium.org/130983011/

BUG=340322

Review URL: https://codereview.chromium.org/146043008

git-svn-id: svn://svn.chromium.org/blink/trunk@166962 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert "Avoid unnecessary style recalc for subtree of focused element."

This CL reverts https://codereview.chromium.org/135183002 because it caused a
regression with the Chrome OS login screen. (See referenced bug.)

TBR=rune@opera.com
BUG=342494

Review URL: https://codereview.chromium.org/154083006

git-svn-id: svn://svn.chromium.org/blink/trunk@166963 bbb929c8-8fbe-4397-9dbb-9b2b20218538