Fix assertion failure in FormAssociatedElement::formRemovedFromTree.

This is a regression by Blink r163680 [1].
FormAssociatedElement::formRemovedFromTree can be called with
m_deletionHasBegun. We should not update the reference counter in it.

Removing RefPtr<> for HTMLFormElement::m_pastNamesMap magically fixes the
original problem.

This CL has no tests. I couldn't make a test which works as a layout test.

[1] http://src.chromium.org/viewvc/blink?revision=163680&view=revision

BUG=326854,328456

Review URL: https://codereview.chromium.org/113423005

git-svn-id: svn://svn.chromium.org/blink/trunk@163945 bbb929c8-8fbe-4397-9dbb-9b2b20218538