Build: Remove DCHECK_ALWAYS_ON flag

* According to libchrome documentation, DCHECK_ALWAYS_ON must be set or
  unset on both our library and libchrome. Flag mis-match may cause
  hard-to-debug segfaults which we experience when using base::Thread

Change-Id: If3740196340b02d2d0bb41e08f0b551d47f49864
Fixes: 73147157
Test: make