Convert some GN test support targets to static libs.

components/storage_monitor/BUILD.gn

Index: components/storage_monitor/BUILD.gn
diff --git a/components/storage_monitor/BUILD.gn b/components/storage_monitor/BUILD.gn
index dbbedf8b0e8ce552f53a2fdc1c48a6ed4e3f3c29..1c008c60c2223dd1e85db7656f86e05f9f758085 100644
--- a/components/storage_monitor/BUILD.gn
+++ b/components/storage_monitor/BUILD.gn
@@ -5,7 +5,7 @@
 import("//build/config/features.gni")
 
 # GYP version: components/storage_monitor.gypi:storage_monitor
-source_set("storage_monitor") {
+static_library("storage_monitor") {
   sources = [
     "image_capture_device.h",
     "image_capture_device.mm",
@@ -88,7 +88,7 @@ source_set("storage_monitor") {
 }
 
 # GYP version: components/storage_monitor.gypi:storage_monitor_test_support
-source_set("test_support") {
+static_library("test_support") {
   testonly = true
   sources = [
     "mock_removable_storage_observer.cc",