Ensure we have the correct thread when allocating obsolete methods.

We were using the incorrect thread to allocate memory for obsolete
methods. This could cause DCHECK failures as one tries to lock a mutex
with a non-running thread.

Bug: 31455788
Test: ./test/testrunner/testrunner.py --host -j40
Change-Id: I650d480a2e6e341509ad1f023398ebcc01913baa
6 files changed