Move WebpDecoder from ios/web to components/image_fetcher

As iOS now use the ImageFetcher from components, the WebPDecoder should also
live in components/image_fetcher/ios.

BUG=683918

Review-Url: https://codereview.chromium.org/2699633006
Cr-Commit-Position: refs/heads/master@{#451596}
22 files changed