Make the mac binary path relative.

This makes the generated build files deterministic with respect to build path.

Bug: 871574
Change-Id: Icf30415f6505710d45743d3921b0ae73b83c6a13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649301Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667248}

build/config/mac/mac_sdk.gni

@@ -75,6 +75,12 @@ if (mac_sdk_path == "") {
 }
 mac_bin_path = find_sdk_lines[1]
 
+# If we're using hermetic Xcode, then we want the paths to be relative so that
+# generated ninja files are independent of the directory location.
+if (!use_system_xcode) {
+  mac_bin_path = rebase_path(mac_bin_path, root_build_dir)
+}
+
 script_name = "//build/config/mac/sdk_info.py"
 sdk_info_args = []
 if (!use_system_xcode) {