Make ThreadableLoader class to use references

ExecutionContext of ThreadableLoader class can not be null.
Move to use references to make caller code safer and remove
extra null checks. Keeping the existing interfaces as is to
create a new CL for changing remaining clients of ThreadableLoader.
Makes this patch smaller and reviewable.

Review URL: https://codereview.chromium.org/204983007

git-svn-id: svn://svn.chromium.org/blink/trunk@170906 bbb929c8-8fbe-4397-9dbb-9b2b20218538