Oilpan: Fix crash in DataObjectTest::TearDown.

As the code comment says, we need to invoke File destructors before restoring
the original Platform object. So we should call Heap::collectAllGarbage. Also we
need Platform::getTraceCategoryEnabledFlag implementation to make TRACE_EVENT0
in GC workable.

BUG=

Review URL: https://codereview.chromium.org/432253002

git-svn-id: svn://svn.chromium.org/blink/trunk@179361 bbb929c8-8fbe-4397-9dbb-9b2b20218538