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- Paths intererpreted by GN such as paths in inputs, sources, lib_dirs,
  etc should either be relative to BUILD.gn, or GN-absolute (relative to
  source root and start with "//").

  They should not be rebased as system-absolute paths; those are only
  needed for paths that are actually outside the source tree.

  If paths are used in multiple contexts, rebasing should be deferred
  until they are used in a context where rebased paths are required.

- Paths passed as arguments to scripts should be rebased on
  root_build_dir, which is the current working directory of the build.

  They should not be rebased on root_out_dir, as that is only the current
  working directory for the default toolchain. Using root_out_dir in
  place of root_build_dir breaks cross compilation.

  They should also not be resolved to system-absolute paths, as that
  introduces an undesired dependency on the location of the source tree.

- The 3rd argument to rebase_path has no effect for GN-absolute paths,
  which includes all paths built by appending to predefined variables
  such as root_out_dir.

Change-Id: I54ff2d9fed0daadacf5bd1bd8fdadc561c3a2882
Reviewed-on: https://chromium-review.googlesource.com/c/1448694
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629579}