Eliminate absolute paths in ninja files for Linux Release build

After this change, the number of steps required for rebuilding all
in Linux Release build after snapshot and "gn gen" is reduced to 384,
the same as touching OUT_DIR/build.ninja.

BUG=723856

Review-Url: https://codereview.chromium.org/2909253002
Cr-Commit-Position: refs/heads/master@{#477584}
2 files changed