base/threading: restrict to set only current thread priority
On some platforms, e.g., Linux, changing other thread's priority is not
permitted by sandbox mechanisms for a security reason.
Since there are no real use cases that really need to change another thread's
priority for now, I'll replace the base API with the one that change the
current thread priority to avoid confusion.
BUG=468793,505474
TEST=base_unittests
TEST=sudo base_unittests --gtest_filter='Thread*'
TEST=git cl try
Review URL: https://codereview.chromium.org/1193303002
Cr-Commit-Position: refs/heads/master@{#338661}
12 files changed
