[Cronet] Clear mock cert verifier reference in CronetEngine.Builder
Ownership of the mock cert verifier is passed to CronetEngine after
CronetEngine.Builder.build() is called. We should clear the reference.
This CL additionally fixes test setup so that
CronetEngine.Builder.build() is only called once for the
life time of a CronetTestFramework.
R=mef@chromium.org
BUG=
557425
Committed:
https://crrev.com/f06e9c86dc630aefee9027cf764b9e898c580372
Cr-Commit-Position: refs/heads/master@{#361510}