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This reverts commit 52e03fa8.

Reason for revert: Some mac builders are broken. e.g. https://ci.chromium.org/buildbot/chromium.fyi/Mac%20Builder%20%28dbg%29%20Goma%20Canary/3941

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}

TBR=michaelpg@chromium.org,lfg@chromium.org

Change-Id: Ib3691d13fb6a787b487c0e11a73e9c0bdaad5b8b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 820604
Reviewed-on: https://chromium-review.googlesource.com/969723Reviewed-by: Shinya Kawanaka <shinyak@chromium.org>
Commit-Queue: Shinya Kawanaka <shinyak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544273}