Bug 1578503 - Enable backdrop-filter by default r=gfx-reviewers,jrmuizel

There are still a few remaining issues with the updated backdrop
filter implementation, specifically:

- We don't use reflectMode yet for blurs (quality issue in some cases)
- Performance may not be optimal in all use cases

However, we can try enabling by default now and work on these as
follow ups.

Differential Revision: https://phabricator.services.mozilla.com/D148684

devtools/shared/css/generated/properties-db.js

@@ -3557,6 +3557,32 @@ exports.CSS_PROPERTIES = {
       "unset"
     ]
   },
+  "backdrop-filter": {
+    "isInherited": false,
+    "subproperties": [
+      "backdrop-filter"
+    ],
+    "supports": [],
+    "values": [
+      "blur",
+      "brightness",
+      "contrast",
+      "drop-shadow",
+      "grayscale",
+      "hue-rotate",
+      "inherit",
+      "initial",
+      "invert",
+      "none",
+      "opacity",
+      "revert",
+      "revert-layer",
+      "saturate",
+      "sepia",
+      "unset",
+      "url"
+    ]
+  },
   "backface-visibility": {
     "isInherited": false,
     "subproperties": [

modules/libpref/init/StaticPrefList.yaml

@@ -7515,7 +7515,7 @@
 # Is support for CSS backdrop-filter enabled?
 - name: layout.css.backdrop-filter.enabled
   type: bool
-  value: false
+  value: true
   mirror: always
 
 # Should stray control characters be rendered visibly?

testing/web-platform/meta/css/css-masking/clip-path/clip-path-svg-text-backdrop-filter.html.ini

@@ -1,2 +0,0 @@
-[clip-path-svg-text-backdrop-filter.html]
-  expected: FAIL