Make SyncCallbackHelper ref counted.

SyncCallbackHelper is unsafely referenced from the success and error callbacks which have lifetime determined by the callback receivers.
This CL allocates SyncCallbackHelper on the heap and uses ref counting to ensure it's alive so long as the callbacks are.

This CL also adds proper tracing of heap-allocated callback arguments when scheduling callbacks for the oilpan build.

BUG=

Review URL: https://codereview.chromium.org/212623011

git-svn-id: svn://svn.chromium.org/blink/trunk@170434 bbb929c8-8fbe-4397-9dbb-9b2b20218538