Revert GC thread changes

This is not quite a straight revery, some manual edits were necessary.

The original CL didn't undergo sufficient design review or testing.  Revert until the regressions can be sorted out.

Bug 8585185


This reverts commit 6dacf8355a0692b52c49f603f43317772cb36175

This reverts commit f8c033db1edf36a0ab09568c3142054f0be2d1a1

Change-Id: Ie7215bdf881332e822603547e92f810f595077fc
5 files changed