Fix Combobox group headers not being associated with items in screenr…

[tomi-msft]

Pull request checklist

• Addresses an existing issue: Fixes Combobox headers not associated with items for screenreader #14906
• Include a change request file using $ yarn change

Description of changes

Cherrypick of #20529

Focus areas to test

(optional)

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit b218568:

Sandbox	Source
Fluent UI Button	Configuration
codesandbox-react-template	Configuration

[size-auditor]

Asset size changes

Project	Bundle	Baseline Size	New Size	Difference
office-ui-fabric-react	office-ui-fabric-react-ComboBox	241.378 kB	241.889 kB	511 bytes

 Over Tolerance (1024 B)  Over Baseline  Below Baseline  New   Removed 1 kB = 1000 B

Baseline commit: 8948edaaaa326fca1d79888abe244674ec204e7f (build)

[fabricteam]

Perf Analysis

No significant results to display.

All results

Scenario	Render type	7.0 Ticks	PR Ticks	Iterations	Status
BaseButton	mount	954	942	5000
Breadcrumb	mount	42508	43046	5000
Checkbox	mount	1560	1537	5000
CheckboxBase	mount	1301	1299	5000
ChoiceGroup	mount	5026	4975	5000
ComboBox	mount	924	898	1000
CommandBar	mount	7754	7697	1000
ContextualMenu	mount	13685	13730	1000
DefaultButton	mount	1150	1144	5000
DetailsRow	mount	3777	3769	5000
DetailsRowFast	mount	3754	3754	5000
DetailsRowNoStyles	mount	3585	3609	5000
Dialog	mount	1734	1726	1000
DocumentCardTitle	mount	1851	1842	1000
Dropdown	mount	2497	2523	5000
FocusTrapZone	mount	1739	1734	5000
FocusZone	mount	1809	1801	5000
IconButton	mount	1747	1757	5000
Label	mount	339	342	5000
Layer	mount	1941	1910	5000
Link	mount	449	443	5000
MenuButton	mount	1476	1491	5000
MessageBar	mount	2082	2038	5000
Nav	mount	3303	3330	1000
OverflowSet	mount	1398	1423	5000
Panel	mount	1491	1464	1000
Persona	mount	776	829	1000
Pivot	mount	1457	1464	1000
PrimaryButton	mount	1271	1290	5000
Rating	mount	7886	7811	5000
SearchBox	mount	1275	1268	5000
Shimmer	mount	2561	2591	5000
Slider	mount	1514	1531	5000
SpinButton	mount	5015	5096	5000
Spinner	mount	417	425	5000
SplitButton	mount	3250	3238	5000
Stack	mount	496	519	5000
StackWithIntrinsicChildren	mount	1666	1665	5000
StackWithTextChildren	mount	4764	4784	5000
SwatchColorPicker	mount	10611	10740	5000
TagPicker	mount	2857	2841	5000
TeachingBubble	mount	51718	51864	5000
Text	mount	451	435	5000
TextField	mount	1397	1426	5000
Toggle	mount	856	834	5000
button	mount	128	109	5000