Add comparison operators for WeakPtr

WeakPtr has 'operator UnspecifiedBoolType() const' for
boolean operations, however having of this operator is only
safe when the == and != operators are declared explicitly, as
otherwise "WeakPtr == WeakPtr" compiles but could do wrong
thing (i.e., convert to UnspecifiedBoolType and then do the
comparison).

This patch adds missing comparison operators for WeakPtr.

Review URL: https://codereview.chromium.org/143403007

git-svn-id: svn://svn.chromium.org/blink/trunk@166827 bbb929c8-8fbe-4397-9dbb-9b2b20218538