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This is a reland of 52e03fa8

Mac compile failure should be fixed with USE_AURA #ifdef around test.

Original change's description:
> AppShell: Suppress context menu in webviews
>
> AppShell doesn't support context menus, but still needs to provide a
> WebViewGuestDelegate to handle the context menu. Otherwise, we reach a
> NOTREACHED() in WebContentsViewChildFrame::ShowContextMenu().
>
> Also removes a WebViewGuestDelegate function that is unused (see
> https://codereview.chromium.org/2836973002).
>
> Bug: 820604
> Change-Id: I0760d397384908a490704f23388e11c4eef113a5
> Reviewed-on: https://chromium-review.googlesource.com/957596
> Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
> Reviewed-by: Lucas Gadani <lfg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#544136}

Bug: 820604
Change-Id: Ib44c00f70a89f88b13f0ba934d725d10353bff86
Reviewed-on: https://chromium-review.googlesource.com/1053181Reviewed-by: Lucas Gadani <lfg@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557741}