Do not show menu items added by extensions inside <webview>.
<webview>'s are supposed to be isolated from regular tab/pages, so
context menus from other extensions should not appear inside them.

BUG=313907
Test=Install some extension that can add a context menu item to some
page, now create an app with <webview> in it and navigate the
<webview> to that page that shows the context menu in regular tab.
Notice that the added context menu item does not appear anymore
inside the <webview>.

Review URL: https://codereview.chromium.org/54973005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232556 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232556
Fix spelling: "supress" -> "suppress"

BUG=165339
TEST=none
TBR=bulach@chromium.org,pkasting@chromium.org

Review URL: https://codereview.chromium.org/49693005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232557 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232557