Fix build: remove DCHECK on TestTaskRunner
Fix build: remove DCHECK on TestTaskRunner

Turns out some tests already depend on leaving uninteresting tasks
on the TestTaskRunner upon destruction. The DCHECK added by the
last CL just causes flakiness. Removing it.

TBR=hjd

Change-Id: I8d1d4d17fbcce9c91dc6518a48594dcaa38519f2

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto