Update a comment in CrossThreadTask.h

Elaborated on why ExecutionContextTask is holding a RawPtr to on-heap objects.

Review URL: https://codereview.chromium.org/385563003

git-svn-id: svn://svn.chromium.org/blink/trunk@177898 bbb929c8-8fbe-4397-9dbb-9b2b20218538