Backport some tests in /content/browser/devtools to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/content/browser/devtools.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

R=caseq@chromium.org

Bug: 891670
Change-Id: I42f549fef6bf7c3e1da3c6b08fdd64353f741a69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786901
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695196}

content/browser/devtools/devtools_video_consumer_unittest.cc

@@ -234,7 +234,7 @@ class DevToolsVideoConsumerTest : public testing::Test {
             weak_factory_.GetWeakPtr()));
   }
 
-  base::test::TaskEnvironment task_environment_;
+  base::test::SingleThreadTaskEnvironment task_environment_;
   base::WeakPtrFactory<DevToolsVideoConsumerTest> weak_factory_{this};
 };