Started at https://chromium-review.googlesource.com/924027.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0a9dcf3d5aa4e5de4ac65b40fb7f85ebfc168eb6

commit 0a9dcf3d5aa4e5de4ac65b40fb7f85ebfc168eb6
Author: Hugo Holgersson <hugoh@vewd.com>
Date: Mon Feb 19 12:34:44 2018

Don't copy hidden selections

Chrome sometimes hides selections*. CTRL+C should not copy
such selections because users expect that CTRL+C has no
effect when their browser doesn't display a selection.

What about cut and paste?
Cut and paste are read/write-operations so they only operate
on _focused_ selections. A focused selection is always visible:

  bool FrameSelection::IsHidden() const {
    if (SelectionHasFocus())
      return false;
    ...

=> Cut and paste already follow the principle of visibility
so we only need to fix "copy". This CL does that.

* See the explainer in crrev.com/525029 for more information.

Bug:  813416 
Change-Id: Ibb5f3868e7a5fc37c7522bb753a41ae9568d98e2
Reviewed-on: https://chromium-review.googlesource.com/924027
Commit-Queue: Hugo Holgersson <hugoh@vewd.com>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537633}
[modify] https://crrev.com/0a9dcf3d5aa4e5de4ac65b40fb7f85ebfc168eb6/third_party/WebKit/Source/core/editing/Editor.cpp
[modify] https://crrev.com/0a9dcf3d5aa4e5de4ac65b40fb7f85ebfc168eb6/third_party/WebKit/Source/core/editing/EditorTest.cpp