skip runtime registration on iOS
skip runtime registration on iOS

These encoders and decoders are not built and linked
in Google3 iOS builds.  We just need to hack a bit to
get DM to build there... we never run it.

Change-Id: I6cdfbb6df3c4fe30096b2715cad004bccec0bcaa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214002
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

HalCanary/skia-hc,google/skia,google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc