Add more components_unittests to GN build

components/precache/core/BUILD.gn

Index: components/precache/core/BUILD.gn
diff --git a/components/precache/core/BUILD.gn b/components/precache/core/BUILD.gn
index ca972252b7973fc99d19c119a48e6ad7e39aff99..4aa14d0ee1f54d779088ebcd63cfeb41913de05c 100644
--- a/components/precache/core/BUILD.gn
+++ b/components/precache/core/BUILD.gn
@@ -18,7 +18,7 @@ config("precache_config") {
   ]
 }
 
-static_library("core") {
+source_set("core") {
   sources = [
     "precache_database.cc",
     "precache_database.h",
@@ -47,3 +47,18 @@ proto_library("proto") {
     "proto/precache.proto",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "precache_database_unittest.cc",
+    "precache_fetcher_unittest.cc",
+    "precache_url_table_unittest.cc",
+  ]
+  deps = [
+    ":core",
+    ":proto",
+    "//testing/gmock",
+    "//testing/gtest",
+  ]
+}