Reviewed by Eric Seidel.

        ApplyStyleCommand::applyRelativeFontStyleChange should take EditingStyle*
        https://bugs.webkit.org/show_bug.cgi?id=50641

        Modified applyRelativeFontStyleChange to take EditingStyle instead of CSSMutableStyleDeclaration.
        Also extracted the logic to obtain the font size delta from ApplyStyleCommand to EditingStyle.
        The font size delta is now stored as a member variable of EditingStyle as supposed to a property
        in CSSMutableStyleDeclration as soon as EditingStyle is instantiated.

        No new tests are added since this is a refactoring.

        * editing/ApplyStyleCommand.cpp:
        (WebCore::ApplyStyleCommand::doApply): Calls applyRelativeFontStyleChange.
        (WebCore::ApplyStyleCommand::applyRelativeFontStyleChange): Takes EditingStyle.
        * editing/ApplyStyleCommand.h:
        * editing/EditingStyle.cpp:
        (WebCore::EditingStyle::EditingStyle): Initializes m_fontSizeDelta.
        (WebCore::EditingStyle::init): Ditto.
        (WebCore::EditingStyle::extractFontSizeDelta): Extracted from applyRelativeFontStyleChange.
        (WebCore::EditingStyle::isEmpty): Takes care of m_fontSizeDelta.
        (WebCore::EditingStyle::setStyle): Calls extractFontSizeDelta.
        (WebCore::EditingStyle::overrideWithStyle): Ditto.
        (WebCore::EditingStyle::clear): Initializes m_fontSizeDelta.
        (WebCore::EditingStyle::copy): Calls extractFontSizeDelta.
        * editing/EditingStyle.h:
        (WebCore::EditingStyle::fontSizeDelta): Added.
        (WebCore::EditingStyle::hasFontSizeDelta): Added.
        * editing/Editor.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75080 268f45cc-cd09-0410-ab3c-d52691b4dbfc