Rename FilterOperation::getOperationType() to type().

This is a simple accessor for m_type. The name 'getOperationType' doesn't follow
our sytle guide.

The function doesn't need to be virtual.

Also, factories of ReferenceFilterOperation, BlurFilterOperation,
DropShadowFilterOperation classes doesn't need to have OperationType argument
because each of them can be only one type.

BUG=none

Review URL: https://codereview.chromium.org/70483003

git-svn-id: svn://svn.chromium.org/blink/trunk@161869 bbb929c8-8fbe-4397-9dbb-9b2b20218538