Add support for additional blend modes to SVGFEBlendElement

New modes to be supported are:

  overlay,
  color-dodge,
  color-burn,
  hard-light,
  soft-light,
  difference,
  exclusion,
  hue,
  saturation,
  color and
  luminosity

Since the new modes don't have any corresponding IDL constants exposed
on the SVGFEBlendElement interface, a mechanism is added to be able to
differentiate between "internal" and "exposed" enumeration values, and
reject those that are "internal-only" from being read or written
through SVGAnimatedEnumeration.

Test feBlend-all-modes.html evolved from test in WebKit commit
http://trac.webkit.org/changeset/170433 by Dirk Schulze.

BUG=389594

Review URL: https://codereview.chromium.org/447193002

git-svn-id: svn://svn.chromium.org/blink/trunk@179732 bbb929c8-8fbe-4397-9dbb-9b2b20218538