[Explore sites]: Refactor navigation delegate to a more common location

mv ntp/ContextMenuManager.java to native_page/
mv suggestions/SuggestionsNavigationDelegate.java to native_page/NativePageNavigationDelegate.java and remove suggestions-only methods
cp suggestions/SuggestionsNavigationDelegateImpl.java to native_page/NativePageNavigationDelegateImpl.java and remove suggestions-only methods
have suggestions/SuggestionsNavigationDelegate.java extend native_page/NativePageNavigationDelegateImpl to contain suggestions-only methods
update all references thereof

Bug: 867488
Change-Id: I64cb9bb6864da2a90b9358ec2a4f339d76df1cdc
Reviewed-on: https://chromium-review.googlesource.com/1232336
Commit-Queue: Cathy Li <chili@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592827}
34 files changed