Cronet: Fix CronetUploadTest.testDestroyAdapterBefore* tests.
The tests weren't actually destroying the adapter when they claimed.
As a result, the final read/rewind call could happen on the "network"
thread between starting the destroy task in Shutdown and actually
tearing down the network thread, at which point the
TestUploadDataStreamHandler network thread pointer is NULL, resulting in
a crash.
The fix is just to destroy the adapters when they should be, at which
point there should be no messages pending on the network thread. May be
worth revising how thread checks are done if more tests are added.
BUG=480623
Review URL: https://codereview.chromium.org/
1098253005
Cr-Commit-Position: refs/heads/master@{#326852}