Add "focus-capturing-application" CaptureStartFocusBehavior

This CL adds support for "focus-capturing-application"
CaptureStartFocusBehavior. It behaves the same as "no-focus-change".

Spec PR: w3c/mediacapture-screen-share#270

Bug: 1480383
Change-Id: Ibd0ce6b1f5fb83f5edc11fc6a7d47ccb5259ceae

screen-capture/getdisplaymedia-capture-controller.https.window.js

@@ -4,7 +4,9 @@
 
 'use strict';
 
-const validFocusBehaviors = ['focus-captured-surface', 'no-focus-change'];
+const validFocusBehaviors = [
+  'focus-capturing-application', 'focus-captured-surface', 'no-focus-change'
+];
 const validDisplaySurfaces = ['window', 'browser'];
 
 test(() => {