Only write gyp-mac-tool when needed
Only write gyp-mac-tool when needed

The gyp-mac-tool is written every time .ninja files are regenerated.
This can cause significant overbuild. This change simply avoids writing
it (and touching its time-stamp) if nothing has changed.

The equivalent fix was done for Windows in crrev.com/1932133002

BUG=607776

Review-Url: https://codereview.chromium.org/1928303003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390983}

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