It seems that it treats mainly a text node in various places, but it's not
guaranteed by any variable declarations. So, first of all, I'd like to make
it clearer.

TextServicesDocument::IsTextNode() isn't necessary because nsINode::IsText()
is enough useful. And AsText() should be zero cost at runtime. So, in blocks
which guarantee specific content node is a text node, this patch appends
AsText() for making the code clearer.

Now, mNode is always a text node, and it may store across "can run script"
boundaries. So, it should be OwningNonNull<Text>.

Depends on D119148

Depends on D119149

Depends on D119150

Depends on D119151

Depends on D119152

Depends on D119153

Depends on D119154

Now, the meaning of OffsetEntry is clear. Therefore, this patch adds comment
explaining the class and its members.

Then, the meaning of TextServicesDocument::mSelStartOffset and
TextServicesDocument::mSelEndOffset becomes clearer since they are used to
create OffsetEntry instances. Therefore, this patch renames them.

Depends on D119155

Now, it stores dom::Text with OwningNonNull. So, once it's leaked, it
wastes a lot of memory spaces. Therefore, we should make mOffsetTable
store UniquePtr<OffsetEntry> instead of OffsetEntry*.

Depends on D119156

There are some methods in TextServicesDocument which work only with
TextServicesDocument::mOffsetTable. Once we move such methods to custom
class of nsTArray<UniquePtr<OffsetEntry>>, we can make TextServicesDocument
simpler.

Depends on D119157

Depends on D119158

Depends on D119159

Depends on D119160

They are indices of OffsetEntryArray. Therefore, they should be managed in it.

Depends on D119161

Depends on D119162

Depends on D119163

Depends on D119164

https://hg.mozilla.org/mozilla-central/rev/ade9a3381ccb
https://hg.mozilla.org/mozilla-central/rev/58608619e491
https://hg.mozilla.org/mozilla-central/rev/3857f3a91edb
https://hg.mozilla.org/mozilla-central/rev/3e43427ec342
https://hg.mozilla.org/mozilla-central/rev/87409043c8f8
https://hg.mozilla.org/mozilla-central/rev/8d3b1cd13597
https://hg.mozilla.org/mozilla-central/rev/790a5005cdbe
https://hg.mozilla.org/mozilla-central/rev/5a72fb7d3da4
https://hg.mozilla.org/mozilla-central/rev/3ba594b4b02d
https://hg.mozilla.org/mozilla-central/rev/42ff08e8e796
https://hg.mozilla.org/mozilla-central/rev/77740303aae0
https://hg.mozilla.org/mozilla-central/rev/622d9feefc23
https://hg.mozilla.org/mozilla-central/rev/0bffa7979736
https://hg.mozilla.org/mozilla-central/rev/3ffbb88fe641
https://hg.mozilla.org/mozilla-central/rev/b771c6d7f21a
https://hg.mozilla.org/mozilla-central/rev/da317e009565
https://hg.mozilla.org/mozilla-central/rev/1232d34c7296
https://hg.mozilla.org/mozilla-central/rev/6af3e577108b