[cast] Stub out call to VideoEncoderImpl::IsSupported on iOS.
[cast] Stub out call to VideoEncoderImpl::IsSupported on iOS.

CL 906403006 introduced a linking error by referencing a symbol that is
not included in iOS builds of the cast sender library. This patch stubs
out the relevant code.

BUG=463713

Review URL: https://codereview.chromium.org/974133002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#318996}

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