Add an instance of SoftwareImageDecodeCache for image decoding for
non-accelerated canvas. This can potentially be used for any other
use-case requiring image decoding on the main thread in blink.

R=chrishtr@chromium.org

Bug: 791828
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I1df793ea00e8e5df26878d6ca8e0769a9ca0c500
Reviewed-on: https://chromium-review.googlesource.com/858570
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555318}