Replace hermetic Xcode installation with CIPD-based flow (take 2)

The 'API' to the scripts remains the same, only the delivery mechanism changes.

This is a reland of https://crrev.com/c/887819.

BUG=797051
R=erikchen@chromium.org, justincohen@chromium.org

Change-Id: I6628170430265e54e6ed0d42509114825b85a6b8
Reviewed-on: https://chromium-review.googlesource.com/1030816Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554882}

build/mac/should_use_hermetic_xcode.py

@@ -30,7 +30,7 @@ def _IsCorpMachine():
 def main():
   allow_corp = sys.argv[1] == 'mac' and _IsCorpMachine()
   if os.environ.get('FORCE_MAC_TOOLCHAIN') or allow_corp:
-    if not mac_toolchain.PlatformMeetsHermeticXcodeRequirements(sys.argv[1]):
+    if not mac_toolchain.PlatformMeetsHermeticXcodeRequirements():
       return "2"
     return "1"
   else: