Add equality and inequality operators for Nodes and TreeScopes

These operators allow comparing Node& or TreeScope& with other references
or pointers. The operators in Document.h are required to allow the compiler
to resolve overloads for document comparisons (since they're both Nodes and
TreeScopes).

Also added operators for comparing RefPtrs to references of the same type
(allowing subtype comparisons does not work for a variety of reasons).

This patch converts all the callsites I could find to use the simplified
operators, thus getting rid of lots of &node->document() and &node->treeScope().

R=ch.dumez@sisa.samsung.com, tkent@chromium.org

Review URL: https://codereview.chromium.org/26887005

git-svn-id: svn://svn.chromium.org/blink/trunk@159486 bbb929c8-8fbe-4397-9dbb-9b2b20218538