[Cronet] Avoid crashing when CronetEngines are created on multiple threads

      Creating a CronetEngine on one thread calls
CronetLibraryLoader.ensureInitialized() which posts to the main thread, but
if a CronetEngine is created immediately after this on the main thread without
first returning to the message loop, that initialization won't have happened
and the initialization of the CronetEngine being created on the main thread
could crash when it tries to get the current native MessageLoop.

BUG=607178

Review-Url: https://codereview.chromium.org/1926683003
Cr-Commit-Position: refs/heads/master@{#390738}