When pre-linking the Cast sender library for the iOS simulator, a number
of symbols are reported as undefined, even though the functions using
them are dead code and the whole chain should disappear under dead code
elimination. The same problem does not occur when targetting iOS
hardware, which suggests a bug in the Intel linker.

This patch either defines those missing symbols or prevents their use.

Specifically, the patch adds dummy definitions for native_library and
process_metrics under iOS which do nothing, and disables the assembly
code path in yuv_convert when compiling for iOS (gyp is unable to build
the simd code properly for target_subarch=both). Otherwise yuv_convert
is functional via its normal CPU code path and is added to the
media_for_ios_cast target to support VideoFrame methods.

BUG=470602
R=miu@chromium.org

Review URL: https://codereview.chromium.org/1030253002

Cr-Commit-Position: refs/heads/master@{#323267}