Compile the physical_shape_layer_unittests.cc TU. (#9187)

This was incorrectly removed by my as I was reworking the GN rules for Fuchsia https://github.com/flutter/engine/pull/8869/files#diff-b6056f6ec9dff658c81f7711f6865e5eL739

flow/BUILD.gn

@@ -114,6 +114,7 @@ executable("flow_unittests") {
     "flow_test_utils.cc",
     "flow_test_utils.h",
     "layers/performance_overlay_layer_unittests.cc",
+    "layers/physical_shape_layer_unittests.cc",
     "matrix_decomposition_unittests.cc",
     "raster_cache_unittests.cc",
   ]