Convert various JNI code to use JavaRef.

No behaviour change intended. Using JavaRef instead of bare jobject
pointers is preferred. These changes are to prepare for an upcoming
refactoring.

Bug: 519562
Change-Id: Iff019df737a9d3f4edae0f5ede8c068cee3443d0
Reviewed-on: https://chromium-review.googlesource.com/c/1255662
Commit-Queue: Richard Coles <[email protected]>
Reviewed-by: Shimi Zhang <[email protected]>
Reviewed-by: David Trainor <[email protected]>
Reviewed-by: Paul Jensen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#595922}
7 files changed