[image_service] Add flags to ImageService for Salient Images

Adds a whole-service level killswitch.

Also adds two flags to let us switch between Suggest-powered entity
images and OptimizationGuide powered Salient images.

No Config object, because the base/ team is imminently adding
base::Feature state caching, removing the performance need for our
own bespoke feature state caching in a Config object.

Bug: b/244507194, 1367485, b/248367751
Change-Id: I45d494e6547df5f205cedc7efd0591d404d6ccb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4257191
Reviewed-by: Alex Gough <[email protected]>
Commit-Queue: Tommy Li <[email protected]>
Reviewed-by: Sophie Chang <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1106924}
diff --git a/components/image_service/BUILD.gn b/components/image_service/BUILD.gn
index 5e67915b..645ae4d 100644
--- a/components/image_service/BUILD.gn
+++ b/components/image_service/BUILD.gn
@@ -5,10 +5,13 @@
 component("image_service") {
   defines = [ "IS_IMAGE_SERVICE_IMPL" ]
   sources = [
+    "features.cc",
+    "features.h",
     "image_service.cc",
     "image_service.h",
   ]
   deps = [
+    "mojom:mojo_bindings",
     "//base",
     "//components/google/core/common",
     "//components/keyed_service/core",