Add support for compiling V8 standalone using gn

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 91ec3caffa06433d9bea5cf1a9eeed7754e69285..e4d7fc4f325a76d329b6bcb33b1d1be2bb56ce1f 100644
--- a/DEPS
+++ b/DEPS
@@ -7,8 +7,12 @@ vars = {
 }
 
 deps = {
+  "v8/build/config":
+    Var("git_url") + "/chromium/src/build/config.git" + "@" + "92c27f8ec14005abd5f9bc99482770d5c6d2d060",
   "v8/build/gyp":
     Var("git_url") + "/external/gyp.git" + "@" + "2889664b9fa88cce175c5c7cdf207d28420a7412",
+  "v8/build/toolchain":
+    Var("git_url") + "/chromium/src/build/toolchain.git" + "@" + "229f311ee66932e66c227b375bf1b2909695b812",
   "v8/third_party/icu":
     Var("git_url") + "/chromium/deps/icu.git" + "@" + "10834e84d250eb31a73496bfdb7923eaa43ead51",
   "v8/buildtools":
@@ -57,6 +61,51 @@ hooks = [
         'v8/build/landmines.py',
     ],
   },
+  # Pull GN binaries. This needs to be before running GYP below.
+  {
+    'name': 'gn_win',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=win32',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'v8/buildtools/win/gn.exe.sha1',
+    ],
+  },
+  {
+    'name': 'gn_mac',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=darwin',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'v8/buildtools/mac/gn.sha1',
+    ],
+  },
+  {
+    'name': 'gn_linux32',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=linux*',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'v8/buildtools/linux32/gn.sha1',
+    ],
+  },
+  {
+    'name': 'gn_linux64',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=linux*',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'v8/buildtools/linux64/gn.sha1',
+    ],
+  },
   # Pull clang-format binaries using checked-in hashes.
   {
     "name": "clang_format_win",