Merge ThreadedInputHandler and InputHandler.

There is no other implementer of InputHandler, so it is an unnecessary
abstraction to separate them.  Also, the name ThreadedInputHandler is
misleading, since we use it even in single-threaded mode.

InputHandlerProxyTest does some yak-shaving to make a MockInputHandler.
(I think we should try to move away from fine-grained mocks like this.)

Bug: 1344941
Change-Id: Iaf7d9d0a19703db43dd72ec42060a4564488570e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3763157
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1025268}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 3eaf28c..9ae3d8c 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -43,6 +43,7 @@
     "input/browser_controls_offset_manager_client.h",
     "input/browser_controls_state.h",
     "input/compositor_input_interfaces.h",
+    "input/input_handler.cc",
     "input/input_handler.h",
     "input/layer_selection_bound.cc",
     "input/layer_selection_bound.h",
@@ -74,8 +75,6 @@
     "input/snap_fling_curve.h",
     "input/snap_selection_strategy.cc",
     "input/snap_selection_strategy.h",
-    "input/threaded_input_handler.cc",
-    "input/threaded_input_handler.h",
     "input/touch_action.h",
     "layers/append_quads_data.cc",
     "layers/append_quads_data.h",