Remove Source/platform/image-encoders/skia
The "skia" is no longer needed since we only use Skia in Blink. Move
the image encoders up a directory level, adjust #includes.
Minor: fix presubmit upload nit in PNGImageEncoder.cpp.
No change in behavior, no new tests.
BUG=
439655
Committed:
https://crrev.com/5f52d011f7437a00026c205d56c96cd7fdeb174e
Cr-Commit-Position: refs/heads/master@{#391756}