Reference written files relatively when possible.
Reference written files relatively when possible.

The intermediately generated files are children of the output build
directory and shouldn't need long absolute path references that
change more often than necessary.

BUG=49621
TEST=Equivalence of build files, see https://code.google.com/p/chromium/issues/detail?id=469621#c3

Review URL: https://codereview.chromium.org/1256043006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#341768}

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