Add support for building "ios_chrome_unittests" with gn.

ios/chrome/ios_chrome.gyp

Index: ios/chrome/ios_chrome.gyp
diff --git a/ios/chrome/ios_chrome.gyp b/ios/chrome/ios_chrome.gyp
index 58d83c0272c56e3943920155a853bc4d6081f325..65bff8af9515095b8af147f13db4ab8c89812896 100644
--- a/ios/chrome/ios_chrome.gyp
+++ b/ios/chrome/ios_chrome.gyp
@@ -8,6 +8,7 @@
   },
   'targets': [
     {
+      # GN version: //ios/chrome/app
       'target_name': 'ios_chrome_app',
       'type': 'static_library',
       'include_dirs': [
@@ -35,6 +36,7 @@
       ],
     },
     {
+      # GN version: //ios/chrome/browser
       'target_name': 'ios_chrome_browser',
       'type': 'static_library',
       'include_dirs': [
@@ -569,6 +571,7 @@
       ],
     },
     {
+      # GN version: //ios/chrome/common
       'target_name': 'ios_chrome_common',
       'type': 'static_library',
       'include_dirs': [
@@ -593,6 +596,7 @@
       ],
     },
     {
+      # GN version: //ios/chrome/browser:injected_js
       'target_name': 'injected_js',
       'type': 'none',
       'sources': [
@@ -606,6 +610,7 @@
       },
     },
     {
+      # GN version: //ios/chrome/common/app_group
       'target_name': 'app_group_common',
       'type': 'static_library',
       'sources': [
@@ -625,6 +630,7 @@
       ],
     },
     {
+      # GN version: //ios/chrome/common/app_group:client
       'target_name': 'app_group_client',
       'type': 'static_library',
       'sources': [
@@ -641,6 +647,7 @@
       ],
     },
     {
+      # GN version: //ios/chrome/common/app_group:main_app
       'target_name': 'app_group_mainapp',
       'type': 'static_library',
       'sources': [
@@ -659,6 +666,7 @@
     ['enable_rlz_support==1', {
       'targets': [
         {
+          # GN version: //ios/chrome/browser/rlz
           'target_name': 'ios_chrome_browser_rlz',
           'type': 'static_library',
           'sources': [