Add more components_unittests to GN build

components/precache/content/BUILD.gn

Index: components/precache/content/BUILD.gn
diff --git a/components/precache/content/BUILD.gn b/components/precache/content/BUILD.gn
index f3f7bd9564f05f7fa7f8103541360705a76554d7..57ca5e2c7f172789bf617e8ab2ed2cc6ac094f40 100644
--- a/components/precache/content/BUILD.gn
+++ b/components/precache/content/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("content") {
+source_set("content") {
   sources = [
     "precache_manager.cc",
     "precache_manager.h",
@@ -17,3 +17,15 @@ static_library("content") {
     "//url",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "precache_manager_unittest.cc",
+  ]
+  deps = [
+    ":content",
+    "//testing/gmock",
+    "//testing/gtest",
+  ]
+}