Bug 1615689 [wpt PR 21808] - [css-pseudo] Fix text selection and drag-and-drop for...

Bug 1615689 [wpt PR 21808] - [css-pseudo] Fix text selection and drag-and-drop for pseudo-elements, a=testonly

Automatic update from web-platform-tests
[css-pseudo] Fix text selection and drag-and-drop for pseudo-elements

Before this patch, if you had a tree like

  <div><div::before>abc</div::before>def</div>

and you tried to select "bc", Chromium would end up selecting "ef" in
some cases. Similarly, trying to drag a text and drop it between "bc",
would end up inserting the text between "ef".

This patch fixes the problem, now trying to select "bc" won't select
anything. And dropping some text between "bc" will insert it between
the "cd".

BUG=1051685

TEST=external/wpt/css/css-pseudo/text-selection.html
TEST=fast/css-generated-content/drag-and-drop.html

The 2nd test is internal because I didn't manage to properly simulate
drag and drop using testdriver.js

Change-Id: Iedd594b46022ed434bdb1462232c717ec08f5304
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2047287
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Oriol Brufau <obrufau@igalia.com>
Cr-Commit-Position: refs/heads/master@{#741909}

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wpt-commits: 410f5ece454de3f304f4ac885be244ed9755ac18
wpt-pr: 21808