Don't attempt to set the window shape on Mac.
Don't attempt to set the window shape on Mac.

setWindowShape now checks that the window is frameless, which it isn't on Mac.
Ideally, we would tie this check to the frameless state directly, rather than
duplicating the platformIsMac check, but there's no API for that.

BUG=b/20270661

Review URL: https://codereview.chromium.org/1086223003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#325389}

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