Reland "mac: roll to 10.13 SDK"

This is a reland of d86a6561

Original change's description:
> mac: roll to 10.13 SDK
> 
> This change causes the default hermetic SDK to be version 10.13 (aka Xcode 9.3).
> This also bumps the minimum build host version to 10.13.2, since that
> is the earliest version on which Xcode 9.3 runs.
> 
> Bug: 780980
> Change-Id: I6da3f7d3a060a22d71d5ee18bb88f51d5aedb0a2
> Reviewed-on: https://chromium-review.googlesource.com/c/1394742
> Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#620032}

Bug: 780980
Change-Id: Iea403cc2dcbfc56ce54a6ad72ca4d2729b520fd0
Reviewed-on: https://chromium-review.googlesource.com/c/1436942
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626106}

build/config/mac/mac_sdk_overrides.gni

@@ -9,14 +9,8 @@ _sdk_min_from_env = getenv("FORCE_MAC_SDK_MIN")
 declare_args() {
   # Minimum supported version of the Mac SDK.
   if (_sdk_min_from_env == "") {
-    mac_sdk_min = "10.12"
+    mac_sdk_min = "10.13"
   } else {
     mac_sdk_min = _sdk_min_from_env
   }
 }
\ No newline at end of file
-
-# Always assert that mac_sdk_min is used on non-macOS platforms to prevent
-# unused args warnings.
-if (!is_mac) {
-  assert(mac_sdk_min == "10.12" || true)
-}

build/mac_toolchain.py

@@ -25,18 +25,12 @@ import sys
 
 # This can be changed after running:
 #    mac_toolchain upload -xcode-path path/to/Xcode.app
-MAC_TOOLCHAIN_VERSION = '8E2002'
+MAC_TOOLCHAIN_VERSION = '9E145'
 
 # The toolchain will not be downloaded if the minimum OS version is not met.
-# 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
+# 17 is the major version number for macOS 10.13.
+# 9E145 (Xcode 9.3) only runs on 10.13.2 and newer.
+MAC_MINIMUM_OS_VERSION = 17
 
 MAC_TOOLCHAIN_INSTALLER = 'mac_toolchain'
 
@@ -54,8 +48,7 @@ STAMP_FILE = os.path.join(TOOLCHAIN_ROOT, 'toolchain_build_revision')
 
 def PlatformMeetsHermeticXcodeRequirements():
   major_version = int(platform.release().split('.')[0])
-  return (major_version >= MAC_MINIMUM_OS_VERSION and
-          major_version <= MAC_MAXIMUM_OS_VERSION)
+  return major_version >= MAC_MINIMUM_OS_VERSION
 
 
 def _UseHermeticToolchain():