Use the given task runner for AssociatedInterfaceProvider instead of per-thread task runner

[Context]
This is a part of an effort to achieve MBI. We want to avoid per-thread task runner as much as possible.

[Previous code]
AssociatedInterfaceProvider::proxy_ was using per-thread default task runner because task_runner was not specified.

[What this CL does]
Use the given task runner for AssociatedInterfaceProvider::proxy_ instead of per-thread task runner.

Bug: 1105403
Change-Id: I1dae802f85f0a9c55e5979ee24297d291a43cd4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462967Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816032}

third_party/blink/common/associated_interfaces/associated_interface_provider.cc

@@ -63,7 +63,8 @@ class AssociatedInterfaceProvider::LocalProvider
 AssociatedInterfaceProvider::AssociatedInterfaceProvider(
     mojo::PendingAssociatedRemote<mojom::AssociatedInterfaceProvider> proxy,
     scoped_refptr<base::SingleThreadTaskRunner> task_runner)
-    : proxy_(std::move(proxy)), task_runner_(std::move(task_runner)) {
+    : proxy_(std::move(proxy), task_runner),
+      task_runner_(std::move(task_runner)) {
   DCHECK(proxy_.is_bound());
 }