Use ScopedTaskEnvironment instead of MessageLoop in RenderViewTest.

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:
https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

This CL also removes RenderViewTest::ProcessPendingMessages().

BUG=708584
TBR=jam@chromium.org

Review-Url: https://codereview.chromium.org/2870633002
Cr-Commit-Position: refs/heads/master@{#470047}
8 files changed