SVG: Move/rename reference management to SVGElement

No change in logic, but rename and move.

I used different naming convensions based on suggestions by fs@opera.com :
"referencingElements" is renamed "outgoingReferences", and
"referencedElements" is renamed "incomingReferences".

- |SVGDocumentExtensions::setOfElementsReferencingTarget|
  moved to |SVGElement::setOfIncomingReferences|
- |SVGDocumentExtensions::addElementReferencingTarget|
  moved to |SVGElement::addOutgoingReferenceTo|
- |SVGDocumentExtensions::removeAllTargetReferencesForElement|
  moved to |SVGElement::removeAllOutgoingReferences|
- |SVGDocumentExtensions::rebuildAllElementReferencesForTarget|
  moved to |SVGElement::rebuildAllIncomingReferences|
- |SVGDocumentExtensions::removeAllElementReferencesForTarget|
  moved to |SVGElement::removeAllIncomingReferences|

Changes to files SVG\w+Element.cpp made by a sed script:
s/document()\.accessSVGExtensions()\.addElementReferencingTarget(this, \([a-Z()\*]\+\))/addOutgoingReferenceTo(\1)/g
s/document()\.accessSVGExtensions()\.removeAllTargetReferencesForElement(this)/removeAllOutgoingReferences()/g
s/document()\.accessSVGExtensions()\.setOfElementsReferencingTarget(this)/setOfIncomingReferences()/g

BUG=378670

Review URL: https://codereview.chromium.org/344883007

git-svn-id: svn://svn.chromium.org/blink/trunk@177039 bbb929c8-8fbe-4397-9dbb-9b2b20218538