Fix computation of caret rect alignment when unicode-bidi: plaintext

In |NGCaretRect::ComputeLocalCaretRectAtTextOffset|, the caret position
corresponding to that text offset is first computed, and then it is
clamped to a bounding box that depends on the line box rect and the
line's container. This bounding box depends on the line's text
alignment, as computed by the |ShouldAlignCaretRight| function.

However, the |should_align_caret_right| variable has an additional
condition, such that if the unicode-bidi CSS property is set to
plaintext, that variable can only be true if the cursor is in an LTR
text fragment, regardless of the text alignment. There are no comments
explaining why this extra condition is needed, and it seems to have
been added as part of https://crrev.com/c/2234323 with no reasoning
attached.

This extra condition makes |should_align_caret_right| be false whenever
the cursor is pointing into RTL text, which can make a difference as
shown in the added test. It is also the underlying cause behind
https://crbug.com/1278559. This patch removes this condition.

Bug: 1349390
Change-Id: I0167a4259bf65a10a3f3694d431bbe5421ab6425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3805880
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Andreu Botella <abotella@igalia.com>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1032492}