arc: Mark ScaleFactor as Extensible

This is in preparation to land
https://chromium-review.googlesource.com/c/chromium/src/+/929945
in a safe manner.

BUG=None

Change-Id: I76a37edddce04edcd8bfc8333a7a6d47dd3f762d
Reviewed-on: https://chromium-review.googlesource.com/935362Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Luis Hector Chavez <lhchavez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539160}

components/arc/common/scale_factor.mojom

@@ -5,6 +5,7 @@
 module arc.mojom;
 
 // Duplicates ui::ScaleFactor enum in order to be accessible on Android side.
+[Extensible]
 enum ScaleFactor {
   SCALE_FACTOR_NONE = 0,
   SCALE_FACTOR_100P,