Remove RenderThread::TaskRunner()

Remove RenderThread::TaskRunner()

Now that base::ThreadTaskRunnerHandle::Get() will always return the
correct task runner on the renderer main thread, there's no need for the
RenderThread::TaskRunner() getter any longer since it will just return
the same value. This patch removes that function as well as its usage.

Note that instead of the following:

    DCHECK(content::RenderThread::Get()->TaskRunner()->RunsTasksOnCurrentThread())

it's more correct to check for:

    DCHECK(content::RenderThread::Get());

because the current render thread is only non-null when running on the
renderer thread itself.

BUG=465354

Review URL: https://codereview.chromium.org/1233763004

Cr-Commit-Position: refs/heads/master@{#339460}