`<mat-progress-bar>` is a horizontal progress-bar for indicating progress and activity.

### Progress mode
The progress-bar supports four modes: determinate, indeterminate, buffer and query.

#### Determinate
Operations where the percentage of the operation complete is known should use the 
determinate indicator.

<!-- example(progress-bar-determinate) -->

This is the default mode and the progress is represented by the `value` property.

#### Indeterminate
Operations where the user is asked to wait while something finishes and it’s 
not necessary to indicate how long it will take should use the indeterminate indicator.

<!-- example(progress-bar-indeterminate) -->

In this mode the `value` property is ignored.

#### Buffer
Operations where the user wants to indicate some activity or loading from the server, 
use the buffer indicator.

<!-- example(progress-bar-buffer) -->

In "buffer" mode, `value` determines the progress of the primary bar while the `bufferValue` is 
used to show the additional buffering progress.

#### Query
For situations where the user wants to indicate pre-loading (until the loading can actually be made), 
use the query indicator.

<!-- example(progress-bar-query) -->

In "query" mode, the progress-bar renders as an inverted "indeterminate" bar. Once the response 
progress is available, the `mode` should be changed to determinate to convey the progress. In
this mode the `value` property is ignored.

### Theming
The color of a progress-bar can be changed by using the `color` property. By default, progress-bars
use the theme's primary color. This can be changed to `'accent'` or `'warn'`.  

### Accessibility
Each progress bar should be given a meaningful label via `aria-label` or `aria-labelledby`.