Disable forwarding tasks to the Blink scheduler

Disable forwarding tasks to the Blink scheduler to avoid some
regressions which it has introduced.

BUG=391005,415758,415478,412714,416362,416827,417608
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/609483002

Cr-Commit-Position: refs/heads/master@{#296916}
RenderThreadImpl::RenderThreadImpl(
    scoped_ptr<base::MessageLoop> main_message_loop)
    : ChildThread(Options(ShouldUseMojoChannel())),
      main_message_loop_(main_message_loop.Pass()) {
  Init();
}

RenderThreadImpl::RenderThreadImpl(
    scoped_ptr<base::MessageLoop> main_message_loop)
    : ChildThread(Options(ShouldUseMojoChannel())),
      main_message_loop_(main_message_loop.Pass()) {
  Init();
}
