GitLab

We currently parse content type header to extract boundary
in WebURLLoaderImpl, while we can do so lazily in ImageResource.
The rest of multipart image processing has been already moved
to Blink.

Bug: none
Change-Id: Ic00bb08f2cb0148ae3425135451a403ef755f964
Reviewed-on: https://chromium-review.googlesource.com/c/1394185Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619837}