Add search_engines and precache components to GN build.

components/precache.gypi

Index: components/precache.gypi
diff --git a/components/precache.gypi b/components/precache.gypi
index ec536238d889f6257d91e8fb84a34b2cc6e211fc..5d28ac0d8187cc32d5e673dcec21a1e0832d9607 100644
--- a/components/precache.gypi
+++ b/components/precache.gypi
@@ -5,6 +5,7 @@
 {
   'targets': [
     {
+      # GN Version: //components/precache/content
       'target_name': 'precache_content',
       'type': 'static_library',
       'dependencies': [
@@ -17,6 +18,7 @@
         '..',
       ],
       'sources': [
+        # Note: sources list duplicated in GN build.
         'precache/content/precache_manager.cc',
         'precache/content/precache_manager.h',
         'precache/content/precache_manager_factory.cc',
@@ -24,6 +26,7 @@
       ],
     },
     {
+      # GN version: //components/precache/core
       'target_name': 'precache_core',
       'type': 'static_library',
       'dependencies': [
@@ -54,6 +57,7 @@
       },
     },
     {
+      # GN version: //components/precache/core:proto
       'target_name': 'precache_core_proto',
       'type': 'static_library',
       'sources': [