Move ThreadedInputHandler into its own file

This is a follow-up to https://crrev.com/c/2347371 which is a mechanical
move of ThreadedInputHandler into its own separate file. There are no
changes from the previous CL.

Bug: 915926
Change-Id: I58e6a82271cb81cd5e9dd906a61808d8486b6cb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347924
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796987}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 7d21e1e5..3305acb5 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -63,6 +63,8 @@
     "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",