[ios] Roll hermetic to Xcode 9.2.

Bug: 
Change-Id: I2f49105035442649e4f1c18e5753fc9aabdbef8f
Reviewed-on: https://chromium-review.googlesource.com/807104
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#522547}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 316396117b2b4fc425ef5dcb037bda6e0f74c238

mac_toolchain.py

@@ -37,7 +37,7 @@ 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 = '9B55'
+IOS_TOOLCHAIN_VERSION = '9C40b'
 IOS_TOOLCHAIN_SUB_REVISION = 1
 IOS_TOOLCHAIN_VERSION = '%s-%s' % (IOS_TOOLCHAIN_VERSION,
                                    IOS_TOOLCHAIN_SUB_REVISION)