Oilpan: HRTF dababase loader thread should be attached to Oilpan

When the main thread passes a pointer of GarbageCollected object to a
HRTFDatabase thread (via postTask), the HRTFDatabase thread creates
a CrossThreadPersistent handle for the pointer. However, this is not
allowed because the HRTFDatabase thread is not attached to Oilpan.

We should attach the HRTFDatabase thread to Oilpan.

This CL removes MutexLocker from HRTFDatabase's destructor since we don't need to clear m_htrfDatabase in the destructor.

BUG=427303

Review URL: https://codereview.chromium.org/685153005

git-svn-id: svn://svn.chromium.org/blink/trunk@185024 bbb929c8-8fbe-4397-9dbb-9b2b20218538