Notify users when tab audio capture is not possible

Tab audio cannot be mirrored on devices without a high-res clock.
We show an issue message in the Cast dialog after tab mirroring starts
in such cases.

Bug: 986403
Change-Id: I199f876658b1caec6b0e2d5d323065ddc4b21467
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1736929
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684579}