Original implementation of jpeg writer didn't compile on iOS at all.
This required clients to exclude some code using defines, which leads to
more complicated code.
Now, instead, jpeg writer will compile but will do nothing on iOS. Clients'
code don't need any additional checks now.
BUG=none
Review-Url: https://codereview.webrtc.org/3004603002
Cr-Commit-Position: refs/heads/master@{#19558}
