Load data URI images in an async way according to spec

According to https://github.com/ResponsiveImagesCG/picture-element/issues/223#issuecomment-69772765
we seem to be the only browser that loads data URIs immediately (as if they were cached).
It was introduced as part of the move to async image loading: https://codereview.chromium.org/200923002

This is now causing issues: https://code.google.com/p/chromium/issues/detail?id=469131

This CL changes this behavior to be spec compliant and load data URIs asynchronously, like other resources.

This is bringing https://codereview.chromium.org/1256233002/ to the Chromium repo.

BUG=514206

Review URL: https://codereview.chromium.org/1492303003

Cr-Commit-Position: refs/heads/master@{#363444}
15 files changed