An <img> with no 'src', no 'alt' but 'srcset' represents nothing

Based on [1], it would seem that we should treat the case where we have
no image candidate as representing nothing - this apparently also
matches what other UAs do.

Simplify the NoImageSourceSpecified helper function a bit. (IsNull
implies IsEmpty and !hasAttribute.)

[1] https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element:the-img-element-5

Bug: 897033
Change-Id: I37ecad8faea2b929e8cf35ce7c746313b2d3b915
Reviewed-on: https://chromium-review.googlesource.com/c/1293574
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#601648}
2 files changed