Synchronize is-valid flag upon programmatic clearing of file.value

If a type=file input element is cleared of its current contents by
setting the "value" property to the empty string, make sure the 'is
valid' flag kept on the input element is synchronized.

Failure to do so could incorrectly make the element pass follow-on
validation -- if it additionally has 'required' set.

R=tkent@chromium.org
BUG=336818

Review URL: https://codereview.chromium.org/132233059

git-svn-id: svn://svn.chromium.org/blink/trunk@166115 bbb929c8-8fbe-4397-9dbb-9b2b20218538