REGRESSION: formaction, formenctype, formmethod, formnovalidatio, and formtarget attributes don't work with input[type=submit]

This is a regression by Blink r152700 [1].
We use a shadow node in button input types since r152700. The code handling
form* attribute for input[type=submit] didn't support cases that shadow nodes
were clicked. We need to process the shadow host.

This bug was not detected by existing layout tests because they used
Element::click() to invoke form submission.

[1] http://src.chromium.org/viewvc/blink?view=revision&revision=152700

BUG=303707
TEST=automated.

Review URL: https://codereview.chromium.org/26209003

git-svn-id: svn://svn.chromium.org/blink/trunk@159003 bbb929c8-8fbe-4397-9dbb-9b2b20218538