[CCA] Use mode switching icon during video fps switching.

Bug: 1024402
Test: Verify switching icon is changed during fps switching.
Change-Id: I53e04841094b5439634a3b3c497bbcb4209cbde7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1915648
Auto-Submit: Kuo Jen Wei <inker@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Commit-Queue: Kuo Jen Wei <inker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715215}

chrome/browser/resources/chromeos/camera/src/js/device/constraints_preferrer.js

@@ -233,7 +233,9 @@ cca.device.VideoConstraintsPreferrer =
       this.setPreferredConstFps_(
           /** @type {string} */ (this.deviceId_), this.resolution_,
           this.toggleFps_.checked ? 60 : 30);
-      this.doReconfigureStream_();
+      cca.state.set('mode-switching', true);
+      this.doReconfigureStream_().finally(
+          () => cca.state.set('mode-switching', false));
     });
   }