cc: Move image decodes from tile manager to a separate file.

This patch moves image decode functionality to ImageDecodeController.
This is intended to make modifications to image decodes simpler.

R=enne, weiliangc
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1246633004

Cr-Commit-Position: refs/heads/master@{#341636}