Make DRT output for FEBlend::m_mode lowercase

This is in preparation for switching FEBlend to use WebBlendMode instead
of BlendModeType - which will enable using compositeOperatorName from
GraphicsTypes.h

BUG=389594

Review URL: https://codereview.chromium.org/419533002

git-svn-id: svn://svn.chromium.org/blink/trunk@178853 bbb929c8-8fbe-4397-9dbb-9b2b20218538