PendingScript, HTMLScriptRunner: tiny fixes.

1) PendingScript.h was missing include Element.h. This error didn't show up
because usually Element.h was included before PendingScript.h. It can be
demonstrated e.g., by making V8ScriptRunner.h include HTMLScriptRunner.h. (The
error is: PassRefPtr.h error: invalid use of incomplete type 'struct
WebCore::Element' etc.)

2) HTMLScriptRunner::frame() was a stray declaration.

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/388433002

git-svn-id: svn://svn.chromium.org/blink/trunk@177828 bbb929c8-8fbe-4397-9dbb-9b2b20218538