Always copy a progressive decoded image in impl-side painting mode

An optimization for impl-side painting was done for fully received
images. We let the decoder to write directly to the memory provided by
Skia.

The optimization missed a corner case. If the image was not received
in one go but received progressively. Then when the image is fully
received the last decode does not write directly to the memory Skia
provided. Hence the image becomes empty.

We should detect for such case and only do this optimization if the
image is fully received in one step.

BUG=346984
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/176923023

git-svn-id: svn://svn.chromium.org/blink/trunk@167932 bbb929c8-8fbe-4397-9dbb-9b2b20218538