Set value of m_textAsOfLastFormControlChangeEvent before triggering change event

Value updated inside change event, get overwritten by setTextAsOfLastFormControlChangeEvent.

New value if similar to previous value was not triggering change event.

R=tkent, keishi
BUG=378871
TEST=OnChange event should be fired, when value is changed inside change
handler and same text is re-entered.

Review URL: https://codereview.chromium.org/313013005

git-svn-id: svn://svn.chromium.org/blink/trunk@175565 bbb929c8-8fbe-4397-9dbb-9b2b20218538