Only allow once GC request at a time.

There is no reason to allow multiple, allocating threads may have
multiple GC requests before the GC starts.

Bug: 17942071

(cherry picked from commit 71891f9da2fbcb865931da70ae759032e6698106)

Change-Id: I5e2162373bcef1027eb71e5b1699186ff015f92d
1 file changed