[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
Committed: https://crrev.com/cc24128062d1f6b34052460bf4b3f7f854b263b4
Cr-Commit-Position: refs/heads/master@{#318996}

[jfroy, 2015-03-04 00:39:05]

jfroy@chromium.org changed reviewers:
+ hubbe@chromium.org, miu@chromium.org

[miu, 2015-03-04 00:54:16]

The CQ bit was checked by miu@chromium.org

[miu, 2015-03-04 00:54:17]

lgtm

[commit-bot: I haz the power, 2015-03-04 00:56:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/974133002/1

[commit-bot: I haz the power, 2015-03-04 02:03:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 02:04:13]

Patchset 1 (id:??) landed as
https://crrev.com/cc24128062d1f6b34052460bf4b3f7f854b263b4
Cr-Commit-Position: refs/heads/master@{#318996}