mat-select doesn't select the focused item on tab-keypress

[saithis]

We switched from an other select implementation to mat-select and now some users are complaining that this is brocken and rightfully pointed out, that the native browser select behaves this way.

Reproduction

Use StackBlitz to reproduce your issue: https://stackblitz.com/angular/xevrkjeylnv?file=app%2Fselect-overview-example.ts

Steps to reproduce:

1. click the basic mat-select
2. use the arrow down key until "Tacos" is marked
3. press the tab key

Expected Behavior

I expected "Tacos" to be selected, as this is the behaviour of a native select and also the mat-select with native select.

Actual Behavior

Previous selected value stays selected (or nothing, if nothing was selected)

Environment

• Angular: 8.2.11
• CDK/Material: 8.2.1
• Browser(s): Tested with Firefox and Chrome, but probably all browsers
• Operating System (e.g. Windows, macOS, Ubuntu): Windows

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