Add a flag for developing the Chromevox Developer Option

Bug: 816369
Test: None
Change-Id: Ide3fe8d28b0e2bc9ada51f78595e0c9271ab260b
Reviewed-on: https://chromium-review.googlesource.com/1160838
Reviewed-by: David Tseng <[email protected]>
Reviewed-by: Yuki Awano <[email protected]>
Commit-Queue: Eriko Kurimoto <[email protected]>
Cr-Commit-Position: refs/heads/master@{#581814}
9 files changed