Add a move constructor and move assignment operator to local, global and
weak references to reduce reference churn (saves a small number of bytes).
Allow weak references to be constructed from JavaRef.
Replace NULL with nullptr in jni_weak_ref code.

BUG=

Review-Url: https://codereview.chromium.org/2268553002
Cr-Commit-Position: refs/heads/master@{#414444}