Oilpan: Don't destruct HRTFDatabaseLoader::m_thread during a sweeping phase

It is not allowed to destruct HRTFDatabaseLoader::m_thread during a sweeping phase
because WebThreadSupportingGC's destructor enters a safe point scope.
This CL avoids the situation by explicitly joining with the loader thread in AudioContext::uninitialize().

In order to make sure that the loader thread finishes touching m_thread,
this CL posts a clean-up task and wait for its completion before calling
m_thread.clear().

BUG=432041

Review URL: https://codereview.chromium.org/711323002

git-svn-id: svn://svn.chromium.org/blink/trunk@185177 bbb929c8-8fbe-4397-9dbb-9b2b20218538