Use ScopedTaskEnvironment instead of MessageLoopForUI in content tests.

ScopedTaskEnvironment allows usage of ThreadTaskRunnerHandle and
base/task_scheduler/post_task.h within its scope. It should be
instantiated in everytest that uses either of these APIs
(i.e. no test should instantiate a MessageLoop directly).

Motivation for ScopedTaskEnvironment can be found in:


Cr-Commit-Position: refs/heads/master@{#468368}
14 files changed