GWT-RPC: don't waste memory doing DEBUG logging when not enabled

ProxyCreator should not set
SerializableTypeOracleBuilder.setLogOutputWriter(..)
if logging level is not reachable from DEBUG.

Bug: issue 8183
Change-Id: I02afbda0bea4e9288eef1cffbc6b7ddc1e68a2a4
Delete DocumentRootImpl

Change-Id: I3394239e8f723453655cbbe60f34ec1bf727cd83
Merge remote-tracking branch 'upstream/master'