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This will allow us to determine how much magic window mode is used vs
presentation by looking at the ratio of this UseCounter vs presentation
UseCounter.

TBR=vollick@ for web_feature.mojom, which doesn't need owner review.
BUG=776086

Change-Id: I6e28433a00a8dbb9be3315d926d8f2b225be5b3e
Reviewed-on: https://chromium-review.googlesource.com/752079Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543541}