Add support for compiling V8 standalone using gn

DEPS

 # Note: The buildbots evaluate this file with CWD set to the parent
 # directory and assume that the root of the checkout is in ./v8/, so
 # all paths in here must match this assumption.
 
 vars = {
   "git_url": "https://chromium.googlesource.com",
 }
 
 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":
     Var("git_url") + "/chromium/buildtools.git" + "@" + "3b302fef93f7cc58d9b8168466905237484b2772",
   "v8/testing/gtest":
     Var("git_url") + "/external/googletest.git" + "@" + "be1868139ffe0ccd0e8e3b37292b84c821d9c8ad",
   "v8/testing/gmock":
     Var("git_url") + "/external/googlemock.git" + "@" + "29763965ab52f24565299976b936d1265cb6a271",  # from svn revision 501
   "v8/tools/clang":
     Var("git_url") + "/chromium/src/tools/clang.git" + "@" + "7ec0c52c7a66cd94c96b8eb37bacd26d152e40e6",
@@ skipping 28 matching lines @@
     # This clobbers when necessary (based on get_landmines.py). It must be the
     # first hook so that other things that get/generate into the output
     # directory will not subsequently be clobbered.
     'name': 'landmines',
     'pattern': '.',
     'action': [
         'python',
         '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",
     "pattern": ".",
     "action": [ "download_from_google_storage",
                 "--no_resume",
                 "--platform=win32",
                 "--no_auth",
                 "--bucket", "chromium-clang-format",
                 "-s", "v8/buildtools/win/clang-format.exe.sha1",
@@ skipping 38 matching lines @@
     'name': 'clang',
     'pattern': '.',
     'action': ['python', 'v8/tools/clang/scripts/update.py', '--if-needed'],
   },
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "pattern": ".",
     "action": ["python", "v8/build/gyp_v8"],
   },
 ]