Add petermarshall@ to OWNERS.
Add petermarshall@ to OWNERS.

Change-Id: Ibbd5b9c526c0b890a54f5b92f2b2d0cc3873f8f3
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2115632
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Commit-Queue: Benedikt Meurer <024a1e0af501338d11c3b837c6193d11c140d792@chromium.org>

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