[ios] Roll hermetic to Xcode 9 beta 6.

Bug: 
Change-Id: I50cc924c577b156f99ba6397b698dc28914550e1
Reviewed-on: https://chromium-review.googlesource.com/629046
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#497216}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 64cf011472fb01efd09628815fb66d469eaa00c1

mac_toolchain.py

@@ -34,7 +34,7 @@ MAC_TOOLCHAIN_VERSION = '%s-%s' % (MAC_TOOLCHAIN_VERSION,
 # 16 is the major version number for macOS 10.12.
 MAC_MINIMUM_OS_VERSION = 16
 
-IOS_TOOLCHAIN_VERSION = '8C1002'
+IOS_TOOLCHAIN_VERSION = '9M214v'
 IOS_TOOLCHAIN_SUB_REVISION = 1
 IOS_TOOLCHAIN_VERSION = '%s-%s' % (IOS_TOOLCHAIN_VERSION,
                                    IOS_TOOLCHAIN_SUB_REVISION)