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The ChromeVox extension was changed recently to call
commandLinePrivate to detect some experimental switches.
This API is not available on ChromeCast and was causing
the extension to fail on init.  Stubbing this API out
so the extension doesn't fail.

Bug: None
Test: Manual, display assistant
Change-Id: I86322b85778ca7d96d66ca90a119eae9347b84f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1786581Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694903}