Bug 1559094 - Restore old behavior for background-size: cover + zero-…

…height background positioning area. r=dholbert

This restores our previous behavior and adds a .tentative test because I
couldn't see how the spec makes any sense.

Filed w3c/csswg-drafts#4049 about that.

Differential Revision: https://phabricator.services.mozilla.com/D35571

UltraBlame original commit: 7ae44e68ef103e451b5a535b4f77407c0ae78382

layout/painting/nsImageRenderer.cpp

@@ -2315,6 +2315,9 @@ CONTAIN
 =
 (
 constrainingRatio
+&
+&
+constrainingRatio
 <
 aIntrinsicRatio
 )

testing/web-platform/tests/css/css-backgrounds/background-size-cover-003-ref.html

@@ -0,0 +1,50 @@
+<
+!
+doctype
+html
+>
+<
+title
+>
+CSS
+Test
+Reference
+<
+/
+title
+>
+<
+style
+>
+body
+{
+margin
+:
+0
+}
+div
+{
+width
+:
+100px
+;
+height
+:
+50px
+;
+background
+:
+lime
+;
+}
+<
+/
+style
+>
+<
+div
+>
+<
+/
+div
+.