[scheduler] Exposing the native BelongsToCurrentThread in Java.

As part of an larger project to unify Java and C++ scheduling I am adding
BelongsToCurrentThread to Java SingleThreadTaskRunner. This will be used
to implement blocking behaviour for the Java methods which require it.

Bug: 863341
Change-Id: I9f471e699608ad70e1e50dd9bbe011f7ac7ae265
Reviewed-on: https://chromium-review.googlesource.com/c/1384046
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618184}
5 files changed