Move toEncodingMimeType function to platform/

This CL is to facilitate OffscreenCanvas convertToBlob() implementation by
putting the toEncodingMimetype function in platform/image-encoders/ so that
both OffscreenCanvas and HTMLCanvasElement can share to use it.

BUG=563830

Review-Url: https://chromiumcodereview.appspot.com/2413813002
Cr-Commit-Position: refs/heads/master@{#426208}
5 files changed