Oilpan: Fix timeout/crash error in speech tests introduced by r172990

The weakly processed hash set was not traced and therefor its backing could become reclaimed prematurely. This CL traces the collection and uses raw pointers and manual weak processing to clean up elements of the collection.

R=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/269663006

git-svn-id: svn://svn.chromium.org/blink/trunk@173194 bbb929c8-8fbe-4397-9dbb-9b2b20218538