Bug 1352643 - Implement multi-position gradient color-stops syntax. r…

…=emilio

This commit adds the multi-position gradient color-stops syntax.

GradientItem::parse_comma_separated is extended to attempt to parse
a LengthOrPercent after each color stop. If it succeeds, it appends
an additional color stop with a duplicate color and the specified
position.

This change is only to the parsing, serialization is left unchanged
as per [1].

[1] w3c/csswg-drafts#2714

Differential Revision: https://phabricator.services.mozilla.com/D7380

UltraBlame original commit: 984902ba6faf1864c04fef525f6764983439ce1b

layout/style/test/property_database.js

@@ -678,6 +678,86 @@ linear
 -
 gradient
 (
+red
+0
+%
+100
+%
+)
+"
+"
+linear
+-
+gradient
+(
+red
+0
+%
+50
+%
+blue
+50
+%
+)
+"
+"
+linear
+-
+gradient
+(
+red
+0
+%
+50
+%
+blue
+50
+%
+100
+%
+)
+"
+"
+linear
+-
+gradient
+(
+red
+0
+%
+50
+%
+0
+%
+blue
+50
+%
+)
+"
+"
+linear
+-
+gradient
+(
+red
+0
+%
+50
+%
+0
+%
+blue
+50
+%
+100
+%
+)
+"
+"
+linear
+-
+gradient
+(
 0
 red
 blue
@@ -1248,6 +1328,86 @@ blue
 )
 "
 "
+radial
+-
+gradient
+(
+red
+0
+%
+100
+%
+)
+"
+"
+radial
+-
+gradient
+(
+red
+0
+%
+50
+%
+blue
+50
+%
+)
+"
+"
+radial
+-
+gradient
+(
+red
+0
+%
+50
+%
+blue
+50
+%
+100
+%
+)
+"
+"
+radial
+-
+gradient
+(
+red
+0
+%
+50
+%
+0
+%
+blue
+50
+%
+)
+"
+"
+radial
+-
+gradient
+(
+red
+0
+%
+50
+%
+0
+%
+blue
+50
+%
+100
+%
+)
+"
+"
 repeating
 -
 radial
@@ -2942,6 +3102,136 @@ red
 blue
 )
 "
+"
+linear
+-
+gradient
+(
+red
+0
+%
+50
+%
+100
+%
+)
+"
+"
+linear
+-
+gradient
+(
+red
+0
+%
+50
+%
+75
+%
+blue
+75
+%
+)
+"
+"
+linear
+-
+gradient
+(
+to
+bottom
+red
+0
+%
+50
+%
+100
+%
+)
+"
+"
+linear
+-
+gradient
+(
+to
+bottom
+red
+0
+%
+50
+%
+75
+%
+blue
+75
+%
+)
+"
+"
+radial
+-
+gradient
+(
+red
+0
+%
+50
+%
+100
+%
+)
+"
+"
+radial
+-
+gradient
+(
+red
+0
+%
+50
+%
+75
+%
+blue
+75
+%
+)
+"
+"
+radial
+-
+gradient
+(
+center
+red
+0
+%
+50
+%
+100
+%
+)
+"
+"
+radial
+-
+gradient
+(
+center
+red
+0
+%
+50
+%
+75
+%
+blue
+75
+%
+)
+"
 ]
 ;
 var