Build changes to allow building chromevox from chromecast

Changes to allow chromecast builds to build the
chromevox extension.  This will eventually eliminate the
prebuilt binary.

Bug: 951459
Test: Manual build
Change-Id: Ie5726d8c4a795182f5311b078732d2d6583efe4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562111
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650842}

• BUILD.gn[diff]
• chrome/browser/resources/chromeos/braille_ime/BUILD.gn[diff]
• chrome/browser/resources/chromeos/chromevox/BUILD.gn[diff]
• chrome/browser/resources/chromeos/chromevox/run_jsbundler.gni[diff]
• third_party/chromevox/BUILD.gn[diff]