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Changed Xcode version in config and OS requirement.

Bug: 1126929
Change-Id: Iece9d00be25f06df812ccfa9a82bdb4df7f85687
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405742Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806277}