Instead of relying on base::InitOnThread to set the initial thread
priority to normal, always set it to thread_params->priority in
ThreadFunc.

The result is that thread priority will be set correctly on all
posix platforms and not only Android.

BUG=584025
TEST=base_unittests --gtest_filter=PlatformThreadTest.ThreadPriorityCurrentThread

Review URL: https://codereview.chromium.org/1660273004

Cr-Commit-Position: refs/heads/master@{#373761}