Add new "Name" type to API that is a supertype of Symbol and String.

Object::SetDeclaredAccessor, Object::SetAccessorProperty, Template::Set,
Template::SetAccessorProperty, and Template::SetDeclaredAccessor now
take a Name as the property name instead of a String.

Add Object::SetAccessor, Template::SetNativeDataProperty, and
ObjectTemplate::SetAccessor overloads that can define accessors for
symbol-named properties.

R=dcarney@chromium.org, rossberg@chromium.org
BUG=v8:3394
TEST=cctest/test-api/TestSymbolProperties
LOG=Y

Review URL: https://codereview.chromium.org/459413002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23247 ce2b1a6d-e550-0410-aec6-3dcde31c8c00