Disable Hermetic Mac toolchain on macOS 10.14+.

The hermetic mac toolchain currently uses Xcode 8. Xcode 8 does not run on macOS
10.14+.

Bug: 904400
Change-Id: I79afe5563631aac5c608c6ef57b594ae5de81e73
Reviewed-on: https://chromium-review.googlesource.com/c/1331907
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607420}

build/mac_toolchain.py

@@ -31,6 +31,13 @@ MAC_TOOLCHAIN_VERSION = '8E2002'
 # 16 is the major version number for macOS 10.12.
 MAC_MINIMUM_OS_VERSION = 16
 
+# The toolchain will not be downloaded if the maximum OS version is exceeded.
+# 17 is the major version number for macOS 10.13. Xcode 8 does not run on macOS
+# 10.14.
+# TODO(https://crbug.com/780980): Once we build with 10.13 SDK, Xcode 9, we
+# should be able to remove this upper bound.
+MAC_MAXIMUM_OS_VERSION = 17
+
 MAC_TOOLCHAIN_INSTALLER = 'mac_toolchain'
 
 # Absolute path to src/ directory.
@@ -46,7 +53,9 @@ STAMP_FILE = os.path.join(TOOLCHAIN_ROOT, 'toolchain_build_revision')
 
 
 def PlatformMeetsHermeticXcodeRequirements():
-  return int(platform.release().split('.')[0]) >= MAC_MINIMUM_OS_VERSION
+  major_version = int(platform.release().split('.')[0])
+  return (major_version >= MAC_MINIMUM_OS_VERSION and
+          major_version <= MAC_MAXIMUM_OS_VERSION)
 
 
 def _UseHermeticToolchain():