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It is a static method which returns a constant ", " string, and which is
only used inside the implementation of DOMWebSocket. Replace it with a
compile-time constant.

Change-Id: I1bb9ab48421394e6b3f759536d53a56888a3af8f
Reviewed-on: https://chromium-review.googlesource.com/1151170Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578519}