Make ConvertableToTraceFormat ref-counted

Previously though TraceEvent is copyable, the copy has side-effect that
will clear the convertable pointers in the source TraceEvent. This
hadn't been a problem before https://codereview.chromium.org/23531042/
which added TraceBuffer::Clone() so exposed the potential problem.

Make ConvertableToTraceFormat ref-counted to allow copying of
TraceEvents.

This also simplifies the client code and trace_event.h because
scoped_refptrs can be assigned just like other types of variables.

TEST=existing tests
BUG=none

Review URL: https://codereview.chromium.org/26266003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227671 0039d316-1c4b-4281-b951-d872f2087c98
Make WeakPtrFactory the last member of IndexedDBQuotaClientTest class

R=dmichael@chromium.org
BUG=303818

Review URL: https://codereview.chromium.org/26571002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227672 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227672