Add a Mock of cc::MutatorHost for testability

Change-Id: Ife1b3c2ca46f19361cf8f92da41528c2c55649c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015343
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Tom McKee <tommckee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735026}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 79ca2276..00ec126 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -482,6 +482,8 @@
     "test/layer_tree_test.h",
     "test/mock_layer_tree_mutator.cc",
     "test/mock_layer_tree_mutator.h",
+    "test/mock_mutator_host.cc",
+    "test/mock_mutator_host.h",
     "test/mock_occlusion_tracker.h",
     "test/paint_op_helper.h",
     "test/pixel_comparator.cc",
@@ -674,6 +676,7 @@
     "resources/resource_pool_unittest.cc",
     "scheduler/scheduler_state_machine_unittest.cc",
     "scheduler/scheduler_unittest.cc",
+    "test/mock_mutator_host_unittest.cc",
     "tiles/checker_image_tracker_unittest.cc",
     "tiles/decoded_image_tracker_unittest.cc",
     "tiles/gpu_image_decode_cache_unittest.cc",