Remove failing ASSERT in HTMLAppletElement::updateWidgetInternal.

This assertion triggers easily in debug builds.
Furthermore, it is not obvious why it is here.

Lacking any explanation of this, it should go away so that it is
possible to debug pages with HTMLAppletElement instances.

BUG=379768

Review URL: https://codereview.chromium.org/364333002

git-svn-id: svn://svn.chromium.org/blink/trunk@177617 bbb929c8-8fbe-4397-9dbb-9b2b20218538