mat-chip with autocomplete can add unwanted chips

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
Typing a partial search term, using arrow keys to highlight option on autocomplete list, and pressing Enter should add only the selected chip.

#### What is the current behavior?
Intermittently, two chips are added. One for the selected value and one with the partial search term.

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

I tried creating a StackBlitz to reproduce it, but couldn't get it to reproduce. Then I realized that it's a race condition causing it. More below...

#### What is the use-case or motivation for changing an existing behavior?
Addtional chip is unexpected behavior

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular/Material >= v6 for sure.

#### Is there anything else we should know?
The problem happens depending on the order that event handlers get executed. The code from the [official example](https://stackblitz.com/angular/klblpljogjd) expects the optionSelected event of the mat-autocomplete to execute first (which clears out the input box), then for the matChipInputTokenEnd event of the input to execute (which avoids adding a chip if it's blank).

For whatever reason, using the exact same code and framework version in my app, the event handlers execute in the opposite order. Since the input hasn't been cleared yet, the search term gets added as an unexpected chip. Then the option from the autocomplete gets added as the expected chip.

Start with the official Angular Material autocomplete and chips example, then trace the matChipInputTokenEnd and optionSelected event handlers and their execution order. Reproduce the race on Angular/Material v6 or later and verify that pressing Enter after selecting an option adds only the selected chip, not the partial search term.

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