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This patch provides the support needed for setting http referrers on URL
loads triggered from the context menu:

- adds Referrer.java holder corresponding to the native
  content::Referrer
- ContextMenuParams.java now holds a Referrer field set (along with
  sanitization) by ContextMenuHelper
- ContextMenuItemDelegate methods that trigger new navigations accept a
  Referrer parameter (save for OpenInNewIncognitoTab, as we drop the
  referrer in this case)
- LoadUrlParams.java also gets a Referrer field, so that embedders can
  set it in their ContextMenuItemDelegate implementations

BUG=340295

Review URL: https://codereview.chromium.org/178193033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255370 0039d316-1c4b-4281-b951-d872f2087c98