Roll mac_toolchain to 796d2b92c

Relevant changes:
  796d2b92c [mac_toolchain] Always run xcodebuild commands in the correct Xcode context
  42953a5b3 [mac_toolchain] Bug fixes for Xcode 10 and DevToolsSecurity

TBR=maruel@chromium.org

Bug: 849728, 846485
Change-Id: Ifbe6af428d609f4530ba6dcb19c16ebab9432af5
Reviewed-on: https://chromium-review.googlesource.com/1088231Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>

cipd_manifest.txt

@@ -14,7 +14,7 @@ infra/tools/luci/vpython/${platform}  git_revision:4d19637ec2c3d1efd8c6a1b052851
 infra/tools/luci/led/${platform}  git_revision:7ba61969ccbe1f9679661555f72ca947da508a70
 
 # Mac toolchain installer
-infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:478dc9dbbaa60dd9827f2b1106c5e70f1f6b429c
+infra/tools/mac_toolchain/${os=mac}-${arch} git_revision:796d2b92cff93fc2059623ce0a66284373ceea0a
 
 # LUCI rpc command line tool
 infra/tools/prpc/${platform} git_revision:e1abc57be62d198b5c2f487bfb2fa2d2eb0e867c