In bug 1925635, I moved the post processing after deleting ranges in
EditorBase::DeleteRangesWithTransaction into the HTMLEditor's override.
At this time, I makes it uses HTMLEditUtils::IsEmptyNode to check whether
the Text and its containers become empty. Therefore, if Ctrl+Backspace
deletes the only word in the Text which starts with a collapsible white-space
causes deleting the Text after deleting all visible characters in the Text.

Therefore, this makes check it with Text::TextDataLength too before searching
the most distant ancestor inline element which becomes empty.

However, it may cause leading invisible white-spaces of the only word visible
because nsFrameSelection computes the word range strictly in the Text.
Therefore, this patch also makes HTMLEditor::DeleteRangesWithTransaction and
AutoDeleteRangesHandler::ComputeRangesToDeleteRangesWithTransaction extend
each range to delete to include surrounding invisible white-spaces too.

Therefore, this patch adds the new method to AutoClonedRangeArray.

Then, I hit 4 existing bugs with new test failures.

One is HTMLEditUtils::LineRequiresPaddingLineBreakToBeVisible. It checks
whether the candidate point to insert a <br> is followed by a block boundary
first. Then, it checks whether the candidate point follows a collapsible
white-space or a block boundary. However, it uses
WSRunScanner::ScanPreviousVisibleNodeOrBlockBoundary() which ignores invisible
white-spaces. Therefore, it will ignore the new invisible white-space and
reaches preceding Text. Thus, it fails to put a <br> and makes the new
invisible white-space "fixed" as invisible.

Therefore, this patch rewrites the check with using
HTMLEditUtils::GetPreviousLeafContentOrPreviousBlockElement().

Next one is, HTMLEditor::DeleteEmptyInclusiveAncestorInlineElements()
returns end of deleted node if it's followed by a non-editable node, i.e.,
an element has contenteditable="false". Therefore, its caller inserts a
<br> to the end of the container when deleting preceding editable node of a
non-editable node.

Therefore, this patch removes the editable state check.

Next, AutoBlockElementsJoiner::HandleDeleteNonCollapsedRange may put a padding
<br> after the moved line, but it does not assume that the moved line does not
ends with a block boundary. This causes failing #46 and #48 tests in
text_bug772796.html. E.g., when pressing Delete in
<div>foo[]<div><span style="white-space:pre"><div>bar</div>baz</span>, we move
the second <div> as a child of the parent <span> to end of the first <div>
like <div>foo<span style="white-space:pre"><div>bar</div></span></div>....
Without the change, it starts to put unnecessary <br> after the <span>`
because of the bug fix in HTMLEditUtils::LineRequiresPaddingLineBreakToBeVisible
above. This result is completely odd from the users point of view (looks like
just move caret), but we should avoid to put the unnecessary <br>.

Finally, we'll fail an assertion when putting caret at the last of
AutoBlockElementsJoiner::DeleteContentInRange because it forgets to flush
the tracking range before using it. This appeared by the changes above.

Therefore, this patch fixes this bug too.