Pull HostThrottler class to host_throttler.h/cc

Bug: n/a
Change-Id: I93d1267165263ebf2527138455c5395faf4ff90c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3340452
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Liu <yjliu@chromium.org>
Auto-Submit: Jun Liu <yjliu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#952624}

diff --git a/ui/aura/BUILD.gn b/ui/aura/BUILD.gn
index d3f8da79..4ce1e31d 100644
--- a/ui/aura/BUILD.gn
+++ b/ui/aura/BUILD.gn

@@ -31,6 +31,7 @@
     "env_input_state_controller.h",
     "env_observer.h",
     "event_injector.h",
+    "host_frame_rate_throttler.h",
     "input_state_lookup.h",
     "layout_manager.h",
     "null_window_targeter.h",
@@ -73,6 +74,7 @@
     "env.cc",
     "env_input_state_controller.cc",
     "event_injector.cc",
+    "host_frame_rate_throttler.cc",
     "input_state_lookup.cc",
     "layout_manager.cc",
     "native_window_occlusion_tracker.cc",