Ensure emma_instr.py always creates expected outputs.
Ensure emma_instr.py always creates expected outputs.

This ensures that running ninja twice on the same target with coverage
enabled results in a no-op build.

BUG=571652

Review URL: https://codereview.chromium.org/1543013002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366653}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk