Move WebContents::DownloadImage/GetManifest to use the new callbacks.

BUG=714018

Change-Id: I330346f3e752870ac1752467ee2c9b934934cb2f
Reviewed-on: https://chromium-review.googlesource.com/974746
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545987}
21 files changed