Use WebBlendMode in FEBlend

This replaces the BlendModeType enumeration with the WebBlendMode for
FEBlend::m_mode. The former is moved to SVGFEBlendElement.
This enables FEBlend to use all of the blend modes available in
WebBlendMode, and leave any subset selection to the client of FEBlend
(which is SVGFEBlendElement).
The BlendModeType->string mapping is replaced by compositeOperatorName().

BUG=389594

Review URL: https://codereview.chromium.org/419253002

git-svn-id: svn://svn.chromium.org/blink/trunk@179029 bbb929c8-8fbe-4397-9dbb-9b2b20218538