Came up during review, no test coverage for partially rendered animated GIF or APNG.  Added the those bugs here to fix that.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b6b96c3de7c6db78762ce660b274514bd18a1c0b

commit b6b96c3de7c6db78762ce660b274514bd18a1c0b
Author: cblume <cblume@chromium.org>
Date: Tue Mar 21 16:43:05 2017

Refactor ImageFrame::setSizeAndColorSpace()

The function ImageFrame::setSizeAndColorSpace does multiple things.
It sets the size and color space & zero fills the data.

We want to be able to not zero fill the entire image. The image decoder
can fill the entire image frame with valid pixel data. And in the case
of partial images, the decoder knows which pixels do not contain image
data, allowing them to zero fill a smaller area.

But for now, just separate the concept of setSizeAndColorSpace() and
zeroFillPixelData().

No change in behavior, no new tests.

R=scroggo@chromium.org
BUG= 701143 

Review-Url: https://codereview.chromium.org/2749703002
Cr-Commit-Position: refs/heads/master@{#458437}

[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/graphics/test/MockImageDecoder.h
[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp
[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/image-decoders/ImageFrame.cpp
[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/image-decoders/ImageFrame.h
[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/image-decoders/bmp/BMPImageReader.cpp
[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/image-decoders/jpeg/JPEGImageDecoder.cpp
[modify] https://crrev.com/b6b96c3de7c6db78762ce660b274514bd18a1c0b/third_party/WebKit/Source/platform/image-decoders/webp/WEBPImageDecoder.cpp