Fix RenderWidgetHostTest input router active state.

This change is a no-op when DropInputEventsWhilePaintHolding feature
is disabled, and makes the test passing when enabled.

Bug: 40057499
Change-Id: I17ffb0bfd16e01bd240198cb8e017abc8a5cd62b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6188131
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Mustaq Ahmed <mustaq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1425973}

content/browser/renderer_host/render_widget_host_unittest.cc

@@ -667,6 +667,7 @@ class RenderWidgetHostTest : public testing::Test {
         TestRenderWidgetHost::CreateStubFrameWidgetRemote());
 
     host_->RendererWidgetCreated(/*for_frame_widget=*/true);
+    host_->input_router()->MakeActive();
   }
 
   void TearDown() override {