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For ease of putting xcode SDK under Chromium src, let me implement
an option to make SDK path relative.

Bug: 1015730
Change-Id: I3c473c3715bfc90b3692649d27ee832048a7e8bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1883469
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710090}