Reviewed by Dan Bernstein.

        - fix storage leak seen on buildbot

        Some other cleanup too. The storage leak was caused by the fact
        that HashTraits<CallIdentifier>::needsDestruction was false, so
        the call identifier objects didn't get deleted.

        * profiler/CallIdentifier.h:
        
        Added a default constructor to create empty call identifiers.

        Changed the normal constructor to use const UString&
        to avoid extra copying and reference count thrash.
        
        Removed the explicit copy constructor definition, since it's what
        the compiler will automatically generate. (Rule of thumb: Either
        you need both a custom copy constructor and a custom assignment
        operator, or neither.)

        Moved the CallIdentifier hash function out of the WTF namespace;
        there's no reason to put it there.

        Changed the CallIdentifier hash function to be a struct rather than
        a specialization of the IntHash struct template. Having it be
        a specialization made no sense, since CallIdentifier is not an integer,
        and did no good.

        Removed explicit definition of emptyValueIsZero in the hash traits,
        since inheriting from GenericHashTraits already makes that false.

        Removed explicit definition of emptyValue, instead relying on the
        default constructor and GenericHashTraits.

        Removed explicit definition of needsDestruction, because we want it
        to have its default value: true, not false. This fixes the leak!

        Changed constructDeletedValue and isDeletedValue to use a line number
        of numeric_limits<unsigned>::max() to indicate a value is deleted.
        Previously this used empty strings for the empty value and null strings
        for the deleted value, but it's more efficient to use null for both.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@38037 268f45cc-cd09-0410-ab3c-d52691b4dbfc