Set content implementation define in public/gpu
Set content implementation define in public/gpu

This will make the component build happy after a source file was added in
this target in https://codereview.chromium.org/1745903002

TBR=posciak

Review URL: https://codereview.chromium.org/1836523003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383308}

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