Revert "[MBI]: Use |render_thread_| instead of RenderThread::Get in AgentSchedulingGroup"

This reverts commit 84d8dbe7.

Reason for revert: 
Suspect causing blink_web_tests failures on WebKi_Linux_ASAN because of heap-use-after-free.

First detected build: https://ci.chromium.org/p/chromium/builders/ci/WebKit%20Linux%20ASAN/18333

blink_web_tests failure:
https://test-results.appspot.com/data/layout_results/WebKit_Linux_ASAN/18333/blink_web_tests/layout-test-results/results.html
---
STDERR: ==1==ERROR: AddressSanitizer: heap-use-after-free on address 0x60d0000065e8 at pc 0x562d5af2328d bp 0x7ffd61c33860 sp 0x7ffd61c33858
STDERR: READ of size 8 at 0x60d0000065e8 thread T0 (content_shell)
STDERR: error: unknown argument '--demangle=True'
STDERR:     #0 0x562d5af2328c in content::AgentSchedulingGroup::RemoveRoute(int) ./../../content/renderer/agent_scheduling_group.cc:133
STDERR:     #1 0x562d5af2328c in ?? ??:0
STDERR:     #2 0x562d5af61740 in content::RenderFrameProxy::~RenderFrameProxy() ./../../content/renderer/render_frame_proxy.cc:248
STDERR:     #3 0x562d5af61740 in ?? ??:0
STDERR:     #4 0x562d5af61b7d in content::RenderFrameProxy::~RenderFrameProxy() ./../../content/renderer/render_frame_proxy.cc:246
STDERR:     #5 0x562d5af61b7d in ?? ??:0
STDERR:     #6 0x562d5af6712d in content::RenderFrameProxy::FrameDetached(blink::WebRemoteFrameClient::DetachType) ./../../content/renderer/render_frame_proxy.cc:620
STDERR:     #7 0x562d5af6712d in ?? ??:0
---
Original change's description:
> [MBI]: Use |render_thread_| instead of RenderThread::Get in AgentSchedulingGroup
>
> R=​haraken@chromium.org, kouhei@chromium.org, talp@chromium.org
>
> Bug: 1111231
> Change-Id: Ia7151587a3459c5262edd5721c63dfec60870cc9
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438709
> Commit-Queue: Dominic Farolino <dom@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Tal Pressman <talp@chromium.org>
> Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#812463}

TBR=haraken@chromium.org,kouhei@chromium.org,dom@chromium.org,talp@chromium.org

Change-Id: I1cda1459c45d129c669fa2736a4ea4322533515c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1111231
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442211Reviewed-by: Takashi Sakamoto <tasak@google.com>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/master@{#812553}

content/renderer/agent_scheduling_group.cc

@@ -118,19 +118,25 @@ AgentSchedulingGroup::~AgentSchedulingGroup() = default;
 // IPC messages to be forwarded to the `RenderThread`, for now. In the future
 // they will be handled directly by the `AgentSchedulingGroup`.
 bool AgentSchedulingGroup::Send(IPC::Message* message) {
-  return render_thread_.Send(message);
+  // TODO(crbug.com/1111231): For some reason, changing this to use
+  // render_thread_ causes trybots to time out (not specific tests).
+  return RenderThread::Get()->Send(message);
 }
 
 // IPC messages to be forwarded to the `RenderThread`, for now. In the future
 // they will be handled directly by the `AgentSchedulingGroup`.
 void AgentSchedulingGroup::AddRoute(int32_t routing_id, Listener* listener) {
-  render_thread_.AddRoute(routing_id, listener);
+  // TODO(crbug.com/1111231): For some reason, changing this to use
+  // render_thread_ causes trybots to time out (not specific tests).
+  RenderThread::Get()->AddRoute(routing_id, listener);
 }
 
 // IPC messages to be forwarded to the `RenderThread`, for now. In the future
 // they will be handled directly by the `AgentSchedulingGroup`.
 void AgentSchedulingGroup::RemoveRoute(int32_t routing_id) {
-  render_thread_.RemoveRoute(routing_id);
+  // TODO(crbug.com/1111231): For some reason, changing this to use
+  // render_thread_ causes trybots to time out (not specific tests).
+  RenderThread::Get()->RemoveRoute(routing_id);
 }
 
 mojom::RouteProvider* AgentSchedulingGroup::GetRemoteRouteProvider() {