Update CompositionAwareMixin to correctly compute composingBase in Web engine (flutter/engine#44139)
It fixes an issue in `CompositionAwareMixin` causing wrong deltas being computed and reported to input clients resulting in wrong state when compositing text in rich text editors like [Fleather](https://github.com/fleather-editor/fleather) and possibly others.
Reported deltas on Mac:
```console
TextEditingDeltaInsertion: start: 0, end: 0, data: k
TextEditingDeltaInsertion: start: 1, end: 0, data: y
TextEditingDeltaReplacement: start: 0, end: 2, data: ãã
TextEditingDeltaInsertion: start: 2, end: 0, data: ã
TextEditingDeltaInsertion: start: 3, end: 0, data: h
TextEditingDeltaReplacement: start: 0, end: 4, data: ãããã¯
TextEditingDeltaReplacement: start: 0, end: 4, data: ä»æ¥ã¯
```
Reported deltas on Web (Before):
```console
TextEditingDeltaInsertion: start: 0, end: 0, data: k
TextEditingDeltaInsertion: start: 1, end: 0, data: y
TextEditingDeltaReplacement: start: 0, end: 2, data: ãã
TextEditingDeltaInsertion: start: 2, end: 0, data: ã
TextEditingDeltaInsertion: start: 3, end: 0, data: h
TextEditingDeltaReplacement: start: 0, end: 4, data: ãããã¯
TextEditingDeltaInsertion: start: 4, end: 0, data: ä»æ¥ã¯
```
Reported deltas on Web (After):
```console
TextEditingDeltaInsertion: start: 0, end: 0, data: k
TextEditingDeltaInsertion: start: 1, end: 0, data: y
TextEditingDeltaReplacement: start: 0, end: 2, data: ãã
TextEditingDeltaInsertion: start: 2, end: 0, data: ã
TextEditingDeltaInsertion: start: 3, end: 0, data: h
TextEditingDeltaReplacement: start: 0, end: 4, data: ãããã¯
TextEditingDeltaReplacement: start: 0, end: 4, data: ä»æ¥ã¯
```
* Fixes https://github.com/flutter/flutter/issues/131335
* Fixes https://github.com/fleather-editor/fleather/issues/150
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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