Add sigbjornf as blink_gc_plugin/ owner.
Add sigbjornf as blink_gc_plugin/ owner.

R=
BUG=
NOTRY=true

Review-Url: https://codereview.chromium.org/1928743002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390325}

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