Add luci-go's isolate tool, downloaded via runhooks.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index ac08cc31f0a34d13ca81b2fc61083dbc94bcf3eb..cc0ac2009a98dec35eccbb632e3b9ee4e043f978 100644
--- a/DEPS
+++ b/DEPS
@@ -658,6 +658,40 @@ hooks = [
                 '-s', 'src/buildtools/linux64/clang-format.sha1',
     ],
   },
+  # Pull luci-go binaries (isolate, swarming) using checked-in hashes.
+  {
+    'name': 'luci-go_win',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=win32',
+                '--no_auth',
+                '--bucket', 'chromium-luci',
+                '-d', 'src/tools/luci-go/win64',
+    ],
+  },
+  {
+    'name': 'luci-go_mac',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=darwin',
+                '--no_auth',
+                '--bucket', 'chromium-luci',
+                '-d', 'src/tools/luci-go/mac64',
+    ],
+  },
+  {
+    'name': 'luci-go_linux',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=linux*',
+                '--no_auth',
+                '--bucket', 'chromium-luci',
+                '-d', 'src/tools/luci-go/linux64',
+    ],
+  },
   # Pull eu-strip binaries using checked-in hashes.
   {
     'name': 'eu-strip',