Several improvements to the behaviour of JavaRef types:

1) Make JavaRef's default constructor public. You can get a null JavaRef
   by conversion from nullptr_t already, so default-constructing one
   seems fine too.
2) Replace templating on a second generic type U with simply JavaRef<T>.
   All existing usage works fine with this; it simplifies the
   implementation and creates fewer ambiguities.
3) Add comments to ScopedJavaLocalRef about the behaviour of the
   constructor and Reset() overloads that take (JNIEnv*, T) as
   parameters, as the current behaviour is confusing but can't yet be
   changed.

BUG=519562

Review-Url: https://codereview.chromium.org/2534973002
Cr-Commit-Position: refs/heads/master@{#435210}