Reviewed by Justin Garcia.

        Assert that editing does not ignore position's anchorNode if position is an offset in anchor
        https://bugs.webkit.org/show_bug.cgi?id=56027

        Added a test to ensure WebKit lets users edit contents inside a button element properly.

        * editing/execCommand/button-expected.txt: Added.
        * editing/execCommand/button.html: Added.
2011-03-17  Ryosuke Niwa  <rniwa@webkit.org>

        Reviewed by Justin Garcia.

        Assert that editing does not ignore position's anchorNode if position is an offset in anchor
        https://bugs.webkit.org/show_bug.cgi?id=56027

        Added the assertion in Position::Position and Position::moveToPosition. This assertion catches
        places where we instantiate positions inside a node on which editingIgnoresContent returns true.

        Test: editing/execCommand/button.html

        * dom/Position.cpp:
        (WebCore::Position::Position): Added an assertion.
        (WebCore::Position::moveToPosition): Ditto.
        * dom/PositionIterator.cpp:
        (WebCore::PositionIterator::operator Position): Avoid creating a position immediately below
        a node whose content is ignored by editing. While this does not avoid creation of positions
        inside ignored contents completely, it works in most cases. Filed the bug 56027 to resolve
        the underlying problem. Without this change, the assertion hits in existing layout tests.
        cannot be tested directly.
        * editing/ApplyStyleCommand.cpp:
        (WebCore::ApplyStyleCommand::addInlineStyleIfNeeded): Call firstPositionInOrBeforeNode
        instead of firstPositionInNode because startNode may as well be a br element. Without this change,
        the assertion hits in existing layout tests.
        * editing/htmlediting.cpp:
        (WebCore::canHaveChildrenForEditing): button is editable so content is not ignored. Added a test
        for this.
        * editing/visible_units.cpp:
        (WebCore::previousBoundary): Added a FIXME.
        (WebCore::startPositionForLine): Because br can also have an inline text box, checking that
        startBox is an inline text box isn't an adequate to instantiate a position inside startNode.
        Call startNode->isTextNode() instead. Without this change, the assertion hits in existing layout
        tests.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@81374 268f45cc-cd09-0410-ab3c-d52691b4dbfc