style: Remove preference "layout.css.shape-outside.enabled".

shape-outside, shape-margin, shape-image-threshold have been shipped in Firefox
62. We can remove the preference.

The change in devtools/shared/css/generated/properties-db.js is generated by
"./mach devtools-css-db"

The actual shape-image CORS mode tests in file_shape_outside_CORS.html are
moved into test_shape_outside_CORS.html because we don't need the <iframe>
trick to enable the feature.

Differential Revision: https://phabricator.services.mozilla.com/D10804

components/style/properties/longhands/box.mako.rs

@@ -604,7 +604,6 @@ ${helpers.predefined_type(
 ${helpers.predefined_type(
     "shape-image-threshold", "Opacity", "0.0",
     products="gecko",
-    gecko_pref="layout.css.shape-outside.enabled",
     animation_value_type="ComputedValue",
     flags="APPLIES_TO_FIRST_LETTER",
     spec="https://drafts.csswg.org/css-shapes/#shape-image-threshold-property",
@@ -615,7 +614,6 @@ ${helpers.predefined_type(
     "NonNegativeLengthOrPercentage",
     "computed::NonNegativeLengthOrPercentage::zero()",
     products="gecko",
-    gecko_pref="layout.css.shape-outside.enabled",
     animation_value_type="NonNegativeLengthOrPercentage",
     flags="APPLIES_TO_FIRST_LETTER",
     spec="https://drafts.csswg.org/css-shapes/#shape-margin-property",
@@ -627,7 +625,6 @@ ${helpers.predefined_type(
     "generics::basic_shape::ShapeSource::None",
     products="gecko",
     boxed=True,
-    gecko_pref="layout.css.shape-outside.enabled",
     animation_value_type="ComputedValue",
     flags="APPLIES_TO_FIRST_LETTER",
     spec="https://drafts.csswg.org/css-shapes/#shape-outside-property",