Prevent surfaces from rendering outside the layer bounds in ChildFrameCompositingHelper.
Prevent surfaces from rendering outside the layer bounds in
ChildFrameCompositingHelper.

BUG=578715

Review URL: https://codereview.chromium.org/1611733003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370608}

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