[ThreadPool] Rename base::ThreadPool to base::ThreadPoolInstance

In order to leave the base::ThreadPool symbol available for
an upcoming "destination" task trait.

Also moved ThreadPoolImpl to be the implementation of TaskExecutor,
not ThreadPoolInstance. It was a mistake that base::ThreadPool was
implementing TaskExecutor, its users shouldn't have access to
PostTask*() and Create*TaskRunner*().

[email protected]
(bypass owners for side-effects beyond //base)

Bug: 968047
Change-Id: I0607fba6d7f30d202bf7f61a9f461b1256e87467
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1634851
Reviewed-by: Gabriel Charette <[email protected]>
Reviewed-by: François Doray <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Auto-Submit: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#664300}
diff --git a/base/observer_list_threadsafe_unittest.cc b/base/observer_list_threadsafe_unittest.cc
index 6d8f4a2..7f3db3e3 100644
--- a/base/observer_list_threadsafe_unittest.cc
+++ b/base/observer_list_threadsafe_unittest.cc
@@ -390,11 +390,11 @@
                           BindOnce(&ObserverListThreadSafe<Foo>::AddObserver,
                                    observer_list, Unretained(&observer_2)));
 
-  ThreadPool::GetInstance()->FlushForTesting();
+  ThreadPoolInstance::Get()->FlushForTesting();
 
   observer_list->Notify(FROM_HERE, &Foo::Observe, 1);
 
-  ThreadPool::GetInstance()->FlushForTesting();
+  ThreadPoolInstance::Get()->FlushForTesting();
 
   EXPECT_TRUE(observer_1.called_on_valid_sequence());
   EXPECT_TRUE(observer_2.called_on_valid_sequence());
@@ -467,7 +467,7 @@
       ->PostTask(FROM_HERE,
                  base::BindOnce(&ObserverListThreadSafe<Foo>::AddObserver,
                                 observer_list, Unretained(&observer)));
-  ThreadPool::GetInstance()->FlushForTesting();
+  ThreadPoolInstance::Get()->FlushForTesting();
 
   observer_list->Notify(FROM_HERE, &Foo::Observe, 1);
   observer.WaitForNotificationRunning();