Use source_set rather than static_library

According to jamesr, static_library is almost never what we want because we
just configure static libraries to be thin archives anyway.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/766053007

engine/src/flutter/compositor/BUILD.gn

@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-static_library("compositor") {
+source_set("compositor") {
   sources = [
     "layer.cc",
     "layer.h",

engine/src/flutter/scheduler/BUILD.gn

@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-static_library("scheduler") {
+source_set("scheduler") {
   sources = [
     "scheduler.cc",
     "scheduler.h",